Understanding Serology and Biological Evidence | Criminal Defense and DNA | Fresno, Ca
Semen - Searching Techniques
Semen is the biological fluid produced by the male sex organs. Semen has a complex composition made up of a cellular component, the spermatozoa, and a fluid component, the seminal plasma. An average ejaculate is 3 to 4 ml and contains 70 to 150 million sperm.
Sperm are the male reproductive cells. Each consists of a head, tail and middle. In humans, the head is a tiny disc, about 4.5 µm long and 2.5 µm wide and contains the DNA. The tail is about 40 µm long, and tends to be rapidly lost after ejaculation. Ape sperm are similar in size and shape to human sperm. Dogs have similar shaped sperm but are different in size to human sperm. Other animals have different shaped sperm that are characteristic to their species.
Seminal plasma contains, cellular materials, proteins, salts, organic substances such as flavins, which are the source of its UV fluorescence, and choline. Some components originate from glands and organs such as the seminal vesicles and the prostate gland, which is the source of acid phosphatase and p30 protein.
In vasectomized males, the surgical procedure involves severing or ligating the ducts carrying sperm to the penis. Therefore vasectomized men will have no sperm but will have the plasma components present in their ejaculate.
After ejaculation, semen is lost from the vagina over time, through the processes of drainage and biochemical change. The tails of the spermatozoa are lost first - the damage beginning immediately after ejaculation. After about 6 hours, about 25% of the spermatozoa will have no tails and by 12 hours, there will be few sperm with intact tails. Twenty four hours after ejaculation there are usually only a few heads left, however these proportions and times are quite variable. The survival time of sperm in stains outside the body depends largely on environmental conditions, but a small stain that has dried quickly may have intact sperm preserved for months or even years.
Visual and Tactile Examination
Semen stains on items of evidence can often be visible to the naked eye. Semen dries to form a white stain on dark surfaces, but may be more yellow in color if dried onto a white surface, or if the stain is old. On either surface, the stain may form a thin crust that imparts a rough or granular texture to the stain. On darker surfaces stains are often easy to locate with the naked eye, however on more patterned surfaces it may be necessary to use an alternative light source to highlight stains or to use a tactile test (crusty touch test) by gently running your fingers (while wearing gloves) across the surface to locate the crusty stains.
Stains not visible to the examiner may be located using additional tools. However, a visual exam should always be performed first, and any suspect stains indicated on the item. This can be done by marking directly on the evidence with a marker or sharpie.
After a preliminary visual and tactile examination, an Alternate Light Source (ALS) may be used to locate less obvious staining. Semen has fluorescing properties due to bacteria (pseudomonas fluorescens) and flavins that are present in semen. The fluorophores present are excited by an energy source (light from the ALS) and emit a light energy rendering the stain visible. Semen is typically visualized best using 400-500 nm wavelengths and using orange safety goggles. An ALS exam must be done in a dark room so that the fluorescence can be seen. Keep in mind that other body fluids such as sweat, urine and saliva can also fluoresce so an ALS exam is not an actual test for semen, it is a searching tool.
Larger items may be searched for stains using a mapping process. Large sheets of damp blotting paper are systematically applied to the item. Papers are applied following a grid like pattern and labeled to indicate their location on the item of interest. Papers are pressed against the item, then removed to a fume hood and sprayed with the color test reagent. This form of searching is used to determine whether semen is present and to determine the distribution of staining on an item. Applying the damp blotting paper against the item results in wetting the original stain and a high risk of transferring the original stain to new areas, especially where layers of fabric overlay each other under the blotted area. To prevent further transfer the analyst should place a sheet of polythene between the upper layer of fabric being tested and the layers underneath. This process can also be done with items of clothing, underwear and large items of bedding.
Once the location of all stains have been identified, the analyst will then begin presumptive testing on these areas. All results should be recorded on the item and in the notes.
Searching techniques provide a lead for further analysis, they should not be substituted for actual testing of evidence for the presence of seminal fluid.
Acid Phosphatase Spot Test
Human semen contains very high concentrations of acid phosphatase, which is an enzyme secreted by the prostate gland. In the body, acid phosphatase is responsible for cleaving a phosphate from phosphorylcholine yielding choline which is important in cellular membrane composition and repair.
Presumptive tests for semen are based on the hydrolysis of phosphate esters and the detection of a liberated organic colored complex. The most common test for acid phosphatase is called the AP spot test. Acid phosphatase, if present in the sample reacts with a chemical called sodium alpha-naphthyl phosphate to form naphthol. Naphthol when combined with Fast Blue B produces a purple azo colored product. This test can be performed in a one step process or a two- step process. In the one step process, the sodium alpha-naphthyl phosphate and the Fast Blue B are combined into one reagent which is applied to a sample of the stain in question. A purple color change within 20-30 seconds (depending on the lab protocol) indicates the possible presence of acid phosphatase in the sample. The two step process differs in that the sodium alpha-naphthyl phosphate (reagent A) and the Fast Blue B (reagent B) are used as separate reagents where one drop is applied of reagent A, then a drop of reagent B to a sample of the stain in question. The same color change reaction is expected with the two step process.
Another presumptive test for acid phosphatase is the BCIP test (5-bromo-4-chloro-3-indolyphosphate). This test is performed by taking a swabbing of the stain in question and adding it to a tube filled with BCIP solution. If acid phosphatase is present it will hydrolyze the BCIP creating a blue color change within ~ 15 minutes.
Both the AP spot test and the BCIP test are presumptive in nature because of the many false positives associated with these tests. Other body fluids can contain other forms of acid phosphatases that may give a false positive reaction such as vaginal fluid (vaginal acid phosphatase is elevated in prepubescent females) and blood (erythrocytic acid phosphatase and seminal acid phosphatase can be elevated in males with prostate cancer). Also, male urine may have acid phosphatase present. Other substances such as plant matter, feminine hygiene products, and spermicides can also give a false positive reaction with the AP tests.
With most forms of biological evidence, a negative reaction with a presumptive test would generally end further testing of that stain. Items in a sexual assault kit (swabs from vagina, cervix, anal, oral and also panties) should be confirmatory tested regardless of the presumptive test results. This is due to the fact that sperm can last longer in the body cavities than acid phosphatase.
Reference information for seminal fluid can be accessed at the following site:http://www.news-medical.net/health/Semen-What-is-Semen.aspx
All AP or BCIP positive samples should undergo microscopic examination to confirm the presence of spermatozoa. The most reliable technique for semen identification is histological staining and light microscopy, since the morphology and dimensions of the human spermatozoon are unique. However, the small sperm can be difficult to locate even using microscopy, since many will have lost their characteristic tails and the heads may be mixed with bacterial and yeast cells present in the sample. The most commonly used stain in the US is Christmas tree stain - named so because of the bright colors it produces. The staining solution includes Nuclear Fast Red to stain the DNA-containing heads bright crimson, and a counter-stain of picric acid - indigocarmine (PIC) that stains the tails blue-gray.
Traditional histological staining with Hematoxylin and Eosin (H&E) is also used, but tends to be more common in European crime lab systems. H&E stain will stain the nuclear material (sperm head) purple and the non nuclear material (tail) pink.
A newer type of stain is a fluorescent stain called SpermPaints that is a fluorescent dye mixed with antibodies which bind to the sperm head and tail antigens. This stain is clear, bright and selective only for sperm cells and will not stain any other debris or cells in the sample. This type of stain is able to be used in conjunction with laser micro-dissection microscopes. These microscopes automatically find sperm cells on the prepared slide and then have the ability to dissect out only the sperm cells using a laser, isolating these cells in a centrifuge tube for later DNA processing.
A test sample is considered to be positive for semen when any number of intact heads and tails are observed on the slide. The presence of one or more heads and no visible tails may also be considered to be a positive result in some laboratories, depending on their policies and criteria for positive sample determination. The issue of intact (with tail attached) vs non intact (without tail attached) sperm has been debated over the years, however, a spermatozoon with or without a tail is still a spermatozoon and any limitations to this means of positive identification lies with the ability of the analyst to identify non intact sperm.
During microscopic examination, you are always looking at the differential staining for the presence of the acrosomal cap on the head and a tail (if attached), the morphology in general of the sperm and its size. Human sperm cells are larger than yeast and bacteria but smaller than free nuclei (from epithelial cells) and dog sperm. Dog sperm are similarly shaped and stain the same as human sperm but are approximately 3 times the size of human sperm.
If, with any piece of evidence, you obtain a positive presumptive test, further positive identification problems may be encountered if the seminal fluid is from a man who has a low sperm count (oligospermic) or who has no spermatozoa present in his seminal fluid (aspermic). When the presumptive Acid Phosphate test indicates the presence of semen, but the microscopic analysis shows no detectable spermatozoa, further tests are carried out to determine the presence of the protein, p30 or prostate specific antigen (PSA) which is only found in high concentration in human semen and is secreted by the prostate gland. It is also called P30 due to its protein molecular weight of 30,000 kD. The function of this protein is that it liquefies semen and aids in dissolving the cervical mucus cap to aid in sperm entry.
Prostate Specific Antigen (PSA)/P30
PSA, or P30, can be detected by precipitin reaction with a specific antiserum using the Ouchterlony process (more on that later). There is also a quantitative immunological test utilizing an enzyme-linked reaction (ELISA). The type of tests most often used today to detect the presence of PSA is based on an antibody-antigen reaction between the sample (PSA) and the antibodies in the test card. It works very similarly to the test previously described for the detection of human hemoglobin (Hematrace®) There are two main test cards that are used for this type of testing: the ABAcard ® P30 test and the Seratec ® PSA Semiquant. Both test cards work similarly, however the Seratec test card has a semi quantitative internal control (set at 4ng/ml) that aids in detecting semen concentration.
Testing for P30 is considered to be a confirmatory test for the presence of semen and is important to use in cases of a vasectomized or aspermic individual to confirm semen when sperm cells would not be present in the sample to confirm via microscopic analysis. P30 can be present in the blood of males with prostatic cancer and also male urine but is not expected to be found in as high concentrations as can be found in semen and would not be detectable with this type of test.
Go to http://www.kumc.edu/instruction/medicine/anatomy/histoweb/male/male25.html, to see microscopic images of intact spermatozoa.
The question of how long was the sperm in the vagina is consistently raised during case investigations or in court. The time since intercourse (TSI) interval is a parameter used to determine whether recovered semen is due to the reported assault rather than from a consensual sexual act that occurred sometime before the alleged assault. Studies have been conducted to investigate the time it takes seminal constituents to leave the vagina and other body orifices, but the results are unclear and difficult to interpret.
In most cases, crime labs don't obtain orifice swabs for analysis in enough time to see motile sperm, but data obtained from hospitals have shown that sperm survive in the vagina for about three hours after ejaculation. However the survival time actually ranges from 1 to 8 hours depending on conditions. Motile sperm seem to survive longer in the cervix and may survive for several hours or several days. Intact spermatozoa have been shown to persist in the vagina for up to 26 hours, and heads for up to 3 days. On rare occasions heads have been seen to persist maybe as long as 7 days.
If abundant spermatozoa can be seen in every microscope field then the swab was probably obtained a day or less after the incident. If the spermatozoa are very few or difficult to find on the slide then intercourse could have occurred up to five days earlier. It must also be remembered though that the presence of spermatozoa can also depend on whether full or partial penetration occurred; whether there was complete ejaculation by the assailant; variations in sperm count, and the victim's activity before or after the incident. Spermatozoa survival in the rectum of living victims has been estimated to be 6 to 65 hours and survival in the oral cavity tends to be much shorter, in the region of 6 hours, since saliva and drinking fluids wash away the sperm.
The level of acid phosphatase in the post coital vagina has been shown to persist for 6 hours to 3 days, and using ELISA techniques or P30 antibody/antigen test cards, most p30 has been shown to be eliminated within 24-27 hours of intercourse.
Saliva is an alkaline fluid that moistens the mouth, softens food, and helps digestion. There are three glands that produce saliva that are located around the mouth under the lower jaw, beneath the tongue, and in front of each ear. These glands are the sublingual, submandibular and the parotid glands. Buccal glands, in the cheeks near the front of the mouth, also secrete saliva. The saliva of the parotid gland contains enzymes called amylases that facilitate the digestion of carbohydrates.
Saliva stains are fairly difficult to locate since they leave a very faint trace when dried and typically cannot be seen with the naked eye. An ALS exam can be performed, but saliva may fluoresce to a much lesser extent than semen.
The test for detecting saliva is the location of the salivary enzyme alpha amylase. Saliva screening depends on the ability of the amylase enzyme to split water-insoluble amylose that has been covalently bound to a dye, to soluble saccharide products that result in a color change. Again, other bodily fluids such as vaginal fluids, breast milk, sweat, blood serum, semen and fecal material may contain some amylase in low concentrations that can result in a weaker test reaction than salivary amylase, producing a false positive result.
The best screening technique (amylase mapping) is similar to that used in searching large items for seminal fluid, using large sheets of blotting paper that have been stained with the test reagent. The dried paper is then moistened with water and pressed onto the area to be tested. The item is then searched in a systematic manner using fresh paper on each area to be tested. The most common reagent used to identify amylase is Phadebas, which are tablets that consist of a blue dye cross -linked to starch (β1-3 glucose bond). In the presence of amylase, the starch is digested, releasing the dye into solution resulting in a blue coloration of the area. A similar test involves the use of Procion Red MX2B amylopectin, also called Lyosine red. In this case the reagent is pink and if the paper comes into contact with saliva, within a few minutes the stain will cause the pink paper to become white in the contact area. Positive tests will infer, but not confirm the presence of saliva
The Phadebas test can also be used quantitatively which may be more sensitive and specific than the mapping process. Using this method, the amylase substrate is cross -linked to starch and made into a tablet. Amylase present in a sample hydrolyses the polymer to soluble blue starch that can be measured colorimetrically at 620 nm. An absorbance value that exceeds 0.3 absorbance units is considered a positive reaction.
Another test for detecting amylase in a sample is the radial diffusion test. This test can give false positive results, especially with mixed fluid stains. The test uses an agarose gel containing 1% starch. An extract of an unknown sample is added to a small hole punched into the gel and allowed to diffuse radially. Amylase present in the sample will hydrolyze the starch in the gel as it diffuses and will produce a clear area on the gel after staining with iodine solution. The size of the clear area is directly proportional to the amount of amylase in the sample.
An alternative to mapping is to take general swabbing from an item where saliva may be indicated from a case scenario OR taking representative cuttings from these areas.
Collecting saliva from bite marks, especially those that are readily visible, is a simple process that involves wiping the area with a moistened cotton swab. The swab can then be air dried and then tested. Bites that do not occur directly on the skin but are inflicted through clothing tend to be a bit trickier. An alternative to mapping is to take general swabbing from an item where saliva may be indicated from a case scenario OR taking representative cuttings from these areas. For example, if the person was bitten on the right breast, samples can be taken from that general area.
Many laboratories do not have procedures for amylase testing. This may due to a number of factors. Saliva is difficult to isolate so not being able to visualize it does not mean it isn't there. People often have varying levels of amylase activity and while one person's saliva may react with an amylase test, another's saliva may give inconclusive or negative results. For these laboratories, a sample is taken from an item of evidence if the case scenario indicates saliva may be present and the sample is extracted for DNA testing.
Saliva in Sexual Assaults
Amylase testing may be considered for certain types of sexual assaults. Saliva is often used as a lubricant in juvenile sexual assaults and may be detected (and further characterized) on underwear. Other situations include oral assault or if the victim was unconscious and does not remember what happened during the sexual assault. Such tests are generally of no value if the victim has showered prior to sample collection.
Prior to screening an item of evidence, it is important to look at the case scenario and talk to the detective about what is needed in the case. Often, trace evidence and hair are important. Many laboratories have separate trace analysts who sweep or remove evidence such as hair, paint ships, soil, gunshot residue, or fibers PRIOR to biological screening to as to avoid the loss of such evidence in the manipulation of the item. If the laboratory does not have a separate analysis for trace evidence, then you must remove this evidence prior to screening the item and place it in a debris fold for possible later analysis.
Urine, feces and vaginal secretions may also be encountered in sexual assaults, sexually motivated homicid Urine
Urine is the net filtrate from the kidneys, characterized by high concentrations of urea and creatinine. Urine has a pale yellow color that comes from the pigment urochrome, a product of urobilinogen or urobilin. Visually locating a urine stain on many surfaces and fabrics can be difficult, but urine does have a very distinctive odor. The odor is ammonia-like and is a result of the bacterial breakdown of urea. Alternative light sources can be useful in finding urine stains since urine fluoresces faint blue-white.
One type of urine screening test depends on colorimetric detection of ammonia produced from urea by urease. There are no confirmatory tests for urine; the presence of urine can only be inferred from a positive result obtained from the presumptive test. The urea litmus paper test indirectly indicates the presence of urea by reacting the sample to urease which would generate ammonia from the urea. The ammonia, which is a basic chemical, can then be indicated using a litmus paper. Another test based on the same reaction is called the Urea-nitrogen test where the ammonia is indicated by the production of a deep blue color reaction.
Urea + H2O ↔ CO2 + 2NH3
Another type of test, which is more commonly used to indicate urine is the Jaffe test, which detects creatinine, a breakdown product of creatine (a component of muscle), that is eliminated through the kidneys. Using the Jaffe test, creatinine forms a red compound with the addition of picric acid.
Microscopic examination of urine relies on the fact that urine contains several solid materials with characteristic crystalline structures, as well as the presence of epithelial cells characteristic to the urinary tract.
Feces are the unabsorbed residues from the gut. They include food residues, gut wall debris, digestive secretions, and bile pigment breakdown products such as urobilinogen.
Feces have a characteristic odor that is mainly due to the chemical compound skatole (which at low levels smells good and is a component of perfumes and colognes). Urobilinogen is a bile pigment excreted in feces and reactions with this compound form the basis of presumptive tests for feces. Urobilinogen can be oxidized to a pink-red product with mercuric chloride; using a 10% solution of mercuric chloride in amyl alcohol as reagent, the suspended sample is centrifuged and the supernatant is treated with zinc chloride. This test became known as the Edelman test, although its reliability has been disputed over the years.
Microscopy is one of the oldest methods for identifying fecal matter. A sample of feces will contain the undigested residues of food digested within the last 12-24 hours. An analyst with sufficient experience can identify characteristic fibrous materials that are found in animal meats, fruits, grains, vegetables and fish.
An experienced parasitologist, using microscopy, may also be able to determine the parasite composition of a fecal sample that can then be compared to the parasite composition of a fecal sample obtained from a suspect.
The presence of enterococci bacteria in the sample is also strong presumptive evidence for the presence of fecal matter.
The test for vaginal secretions is presumptive in nature as well. Vaginal epithelial cells contain a high amount of glycogen. Other cells in the body also contain glycogen such as oral and anal epithelial cells. By exposing a smear of the questioned vaginal secretions to iodine vapor, the glycogenated cells will stain a chocolate brown color. The non-glycogenated cells will be a yellow gold color. This test is called Lugol's test. This test is highly subjective since a positive result is called when the majority of cells stain chocolate brown, however other types of cells may contain this high amount of glycogen. Because of this, the Lugol's test is not widely used for casework purposes.
es, and incidents involving sodomy or bestiality.
The types of testing available for hair evidence are either comparative or biological. For comparative hair testing, physical and chemical characteristics can be determined and compared to a known hair sample from an individual. This type of analysis is typically performed by a trace examiner. Hair can also contain a root which may have suitable amounts of DNA for characterization or the shaft may be used for mitochondrial DNA testing. For biological hair screening you should follow these steps:
- Determine if the hair is human or animal.
- Determine the hair stage
- If there is not adequate root material - is mitochondrial DNA testing a possibility?
Human hairs can be distinguished from animal hairs. Human hairs are generally consistent in color throughout the length of the shaft, the diameter of the shaft is continuous and the medulla of the hair is amorphous or absent in human hairs. Animal hair typically has color banding, the diameter of the shaft is not consistent and the medulla is continuous and takes up more than a third of the diameter of the hair.
Hair growth occurs in three phases: anagen, catagen and telogen. The anagen phase is active hair growth. It contains nucleated cells in the root and in the surrounding sheath material and is generally suitable for nuclear DNA analysis.The catagen phase is the transitional phase after active hair growth. Cell division stops in this phase. It has a characteristic club appearance of the root and may be suitable for nuclear DNA analysis. The telogen phase is the shedding phase. Telogen hairs without follicular tissue may not be amenable to nuclear DNA analysis because of the lack of nucleated cells but may contain sufficient mitochondrial DNA in their roots and hair shafts for analysis. The average person sheds up to 100 hairs per day.
Wear Areas & Touch Evidence
Wear areas are the areas in a piece of evidence such as clothing that can aid in determining the “wearer” of that item through further characterization. These areas may contain cells found in sweat or skin cells that have come off from the friction of wearing that item. This type of evidence is typically collected by swabbing the area or taking a cutting for further characterization.
Locard's Exchange Principle states that when there is contact between two items, there will always be an exchange of material. Touch DNA samples have become more and more prevalent in DNA casework as DNA typing becomes more sensitive to lower levels of DNA that may be left in the execution of a crime. This section gives an overview of touch DNA and describes the procedures for collecting touch DNA samples.
What is Touch DNA?
Evidence that would contain DNA left behind from a person touching or handling an item. The DNA left behind is from epithelial (skin) cells that slough off the skin
Examples: swabs from steering wheels, gun handles, door handles, shell casings
NOTE: Cigarette butts or swabs from cans, bottles or straws that may contain saliva; and swabs from clothing such as shirts or hats that contain DNA from skin cells present on the item from prolonged contact (i.e. wearing the item) are NOT considered touch DNA samples.
Factors in collecting samples for touch DNA
When collecting samples for touch DNA, typically a moist swabbing method is employed. This involves adding a small drop of sterilized water to your swab and then swabbing the surface of the item. Often, a dry swab is used to swab the item after the wet swab. There are a number of different factors that play a role in how you may go about sampling an item for touch DNA. These include, but are not limited to, the following:
- Surface type
If the surface being swabbed is rough/textured, one or two moist swabs may be the best method. You may have to be careful that the swab does not become “shredded” from swabbing the rough surface. If the surface appears wet after you swabbed it, you may want to follow up with a dry swab to collect any residual DNA left behind from the wet swab.
- Size of surface
A large surface area may require more swabs to be taken from that surface. Remember to always keep the number of swabs to a minimum and never use gauze pads to take a swabbing for touch DNA as it spreads the minimal sample over a large surface area on the gauze – making it difficult to obtain adequate sample from the gauze during DNA analysis.
- How dirty the surface is
Sometimes the surface being swabbed is oily or dirty (i.e. guns, shell casings). A wet swab followed by a dry swab may be the best technique for this type of evidence.
- Latent Prints Request on the same surface
When this occurs it is most likely an either/or situation. Either the surface is swabbed for touch DNA collection (and therefore obliterating the possible latent prints) OR it is just processed for latent prints. Each laboratory will have its own policy regarding these issues. Some labs always send an item that has a request for both DNA and Latent Prints to the DNA section first. Other labs will send it to Latent Prints first and the DNA section afterward.
In touch DNA situations the evidence is a bit different from a body fluid such as blood on an item of evidence. If you have blood on a knife – it can be processed for Latent Prints and still be viable for DNA testing. However, with touch DNA, you are already starting with very limited sample. Any processing (superglue, dyes, etc) for the Latent Print processing can dilute out the existing sample.
You must have open communication with the Latent Print Examiner on the case. If it is a case where the item can either be tested for touch DNA OR have a Latent Print exam done – find out how often they are able to develop prints from that type of evidence. Know how often your lab has developed DNA profiles from that type of evidence. The detectives rely on you to be the expert and you often will be driving the decision-making regarding these types of samples.
- Presence of other sources of DNA on the item
An item that has blood or another body fluid that contains high concentrations of DNA should not be tested for touch DNA (unless the body fluid is in a separate area than the area being swabbed for touch DNA). The amount of DNA in the body fluids will overwhelm any DNA present from a person handling or touching that item. Swabbing a bloodstain in the hopes of collecting touch DNA as well is futile.
If you are swabbing an item for both body fluids and touch DNA on separate areas of the item be careful to:
- Swab the areas separately
- Do not allow your swabs to come into contact with another – package them separately
No matter what types of samples you are collecting or testing, talking to the detectives on the case is essential. It is important for them to understand the possible limitations of a request for touch DNA and that people are realistic in their expectations for probative results from this type of sample.
As an evidence examiner, it is important to know about these types of evidence and the role they may play in a crime. Also it is important to note that body fluids such as blood, semen and saliva typically have more DNA present in the stain for further characterization than wear areas and touch areas.
Sometimes, more than one type of analysis is needed on an item of evidence. Other types of analysis besides biological analysis include firearms, latent prints and trace evidence. Communication with the case investigator is important in prioritizing the testing/collection of probative evidence based upon the case history.
Prior to screening an item of evidence, it is important to look at the case scenario and talk to the detective about what is needed in the case. If trace evidence may be important in the case, many laboratories have separate trace analysts who sweep or remove evidence such as hair, paint ships, soil, gunshot residue, or fibers PRIOR to biological screening to as to avoid the loss of such evidence in the manipulation of the item. If the laboratory does not have a separate analysis for trace evidence, then you must remove this evidence prior to screening the item and place it in a debris fold for possible later analysis.
For cases requiring latent print analysis, special care MUST be taken in the initial screening of that evidence. You must avoid smearing or removing possible latent prints during your analysis. This can be done by wearing latent gloves (they are cotton, not latex) or avoid touching too much. Upon visual examination, attempt to visualize any possible prints and stay away from that area on the evidence. Also avoid excessive swabbing or sampling from these pieces of evidence (i.e. you wouldn't i.e. you wouldn't want to do a general swab of the entire surface of a gun if latent prints were also needed). In order to screen the evidence for blood, semen or saliva you should have the evidence first BEFORE it goes to the latent print section since the chemicals and processes to develop prints can affect or remove biological stains.
Some evidence may need firearms analysis which can include tool mark or gunshot residue analysis. If gunshot residue analysis is needed, that analysis needs to be done FIRST before biological screening is done in order to avoid losing the residue during evidence handling. If tool mark or firearms analysis is needed, the item should be screened prior to this type of analysis. Serial number restoration typically involves chemicals which may affect stains. Also the handling (i.e. shooting) of a gun could affect stains as well.