The Best Body Fluids for Forensic DNA Results
When you’re writing your crime fiction book and you’re using forensic DNA evidence as part of your homicide investigation (or other criminal act), it would be helpful to know what types of samples you can get DNA from and have a general idea of their success rates.
Are there certain types of samples that are better for forensic DNA? Are there situations where you can’t get good DNA results? What causes DNA to “go bad”?
As a former forensic DNA analyst, I have examined thousands of items, and I can tell you that not all body fluids are created equal. In my experience, some body fluids provide more consistent DNA results than others (for example, blood vs touch DNA). Knowing these trends can help you use forensic DNA evidence more realistically in your books.
In this post, I list different sources of forensic DNA, provide a rank for each of these sources, and describe situations that might prevent forensic DNA results.
In general, pretty much anything inside of your body (bones and tissues) or anything that can come out/off of you (fluids, skin cells, and hairs) is an option for forensic DNA testing. The following is a list of different body fluids and biological materials that can potentially be analyzed in a crime lab.
- Skin cells/Touch DNA
- Various body tissues (pieces of organs)
- Other secretions (vaginal fluid, tears, pus, mucus, etc.)
However, some of these items are rarely seen in crime labs, or some labs don’t have protocols for certain body fluids. Every procedure that is used in a forensic DNA lab must go through a rigorous validation process; if certain items are rarely seen, the lab might not have a test for it. For example, at my old lab, we did not have specialized protocols for feces or teeth, but I know of several crime labs that do.
DNA Results from Body Fluids
Before I describe typical DNA results from body fluids, I need to add in a little disclaimer: These descriptions are based on my 7 years of experience as a forensic DNA analyst in a state crime lab using the procedures at that lab. While I believe my opinions are similar to other crime labs, it is possible that other labs may have different experiences. In addition, I don’t comment on certain body fluids because I don’t have personal experience with them.
The information below should be used as a general guideline. Ultimately, how much DNA is present and the quality of that DNA determine whether you get a profile.
For each body fluid below, I have include a High, Medium, or Low ranking, which indicates the likelihood of getting DNA results from that specific body fluid.
If I was officially ranking the body fluids, I’d put blood at the top. If I saw a visible stain on an item of evidence and it was of a decent size (1-2 millimeters), I was pretty confident that I would get a DNA profile. Sometimes I’d get high velocity spatter (little dots of blood), which was a little less consistent, but in general, blood is a good source of DNA. (Fun fact: Your red blood cells do not have a nucleus and therefore do not have DNA. The DNA in blood comes from the white blood cells.)
For semen, the main source of DNA is the sperm cells. (Quick note: Semen is the body fluid; sperm are a component of semen.) Semen is a special body fluid, in that there is a procedure that DNA analysts can use to separate the sperm cells from other cells in the sample. This is important because sperm are often mixed with other body fluids (e.g., vaginal secretions). The ability to separate the sperm DNA (male) from the non-sperm DNA into two separate samples is very helpful when interpreting the DNA profiles (mixtures are harder to interpret).
In general, the success rate for sperm is pretty decent if there are enough sperm present. (We actually look for them under the microscope.)
However, you may run into problems if the male has been vasectomized (no sperm) or has a low sperm count. When you separate the male and non-sperm (typically female) portions of a sample from a vasectomized male, there won’t be any sperm in the male sample. Any non-sperm male DNA present will be mixed in with the female portion of the sample, which complicates interpretation. Getting an interpretable male DNA profile in a sample from a vasectomized male is not impossible, but it is much less likely.
Saliva is a decent sample type for successful DNA results on certain items. Cigarette butts and swabs from the mouths of drink containers usually produce good results. Body swabs or saliva on clothing or bedding are a little more iffy.
Saliva is very difficult to search for when you’re examining clothing or bedding, so collection can also be a factor.
There is no specific identification test for sweat, so it is often encountered when a DNA analyst is trying to identify who left items of clothing at crime scenes. An analyst would swab an area of the clothing where sweat would likely be, for example, a ball cap (the inside rim) or a t-shirt (the underarms and/or neck). In my personal opinion, the DNA results for sweat are better than touch DNA (see below) but not as good as saliva. In addition, if the item was worn by multiple people, mixture profiles are very likely.
Hairs (Medium, if skin tag present)
If hairs are in the active growth stage and have a skin tag (i.e., they were pulled out), then the likelihood of getting a profile is pretty good, but results can be inconsistent. The pigments in hairs can sometimes interfere with DNA analysis. Hairs without a root have zero chance of getting DNA results. Hairs without roots have to undergo mitochondrial DNA analysis, which is a specialized type of forensic DNA test not often performed in traditional crime labs. For more information on hairs and how to use them in your crime fiction novel, you can check out this post.
Skin Cells/Touch DNA (Low)
During my employment in the crime lab, I performed a study to see how often you get DNA profiles from gun swabs. (We got A LOT of gun swabs.) This initial gun swab study expanded to which part of the gun produced better results (grip), which then expanded to the success rates of other touch DNA items. The results of this study allowed us to modify our laboratory submission protocols and helped us decide how to prioritize the evidence in our cases.
Certain touch DNA items tend to produce profiles more consistently. Clothing items worn by a suspect and/or handled for a long time and items with rougher surfaces have a better chance of getting a profile. Examples of these are steering wheels and gearshifts in cars and gloves.
Items that are small, nonporous, touched just once, or exposed to the elements tend to have less successful results. Examples include washers/nails, wrenches, door handles, keys, and external car door handles. And just because these items have lower success rates doesn’t mean you can’t ever get a profile. If the DNA belongs to someone who is a “shedder” or if the person was sweating when he or she touched the item, the likelihood of getting a profile is higher.
In a nutshell, the results from touch DNA are very inconsistent. You can get a great profile from a pair of gloves in one case, then nothing in the next case. Low-level mixtures with lots of people are also very common, which are the worst profiles for interpretation.
I put “unknown” here because I do not have enough experience with these types of biological materials to give them a ranking. I only analyzed urine once (and I can’t remember the results), and I never analyzed feces (which I am not sad about. Ha!). My coworker had to examine poo once (vandalism case), and she ended up not getting a DNA profile. Feces has a lot of bacteria and other waste products in it that might interfere with DNA testing. However, I do know of other crime labs that have specialized protocols for urine and feces, so it is possible to get DNA from these types of evidence.
Bones/Teeth (High, depending)
I never analyzed bones or teeth, but my coworker had to test the bone marrow from a femur from a set of unidentified human remains, and she obtained DNA results. Bones and teeth are often sent to specialized labs for mitochondrial DNA testing. Several of my old grad school classmates work at the UNT Center for Human Identification, where they examine this type of evidence all the time.
I analyzed vomit only once and didn’t get a DNA profile. Stomach acid can degrade DNA and possibly interfere with the DNA process.
Tissue isn’t a common type of biological material at crime labs, but it’s an excellent source of DNA. It contains tons of cells. My coworker once had to process a piece of liver tissue as a reference sample, and she got so much DNA the sample had to be diluted.
Other Secretions (High)
Body fluids such as mucus from a tissue or secretions from a Band-Aid usually yield DNA profiles because of the high concentration of cells.
When Good DNA Goes Bad
Now that we have a general idea on which body fluids and biological materials are ideal for forensic DNA testing, let’s talk about what might interfere with getting optimal results.
Items for DNA testing need to be packaged in a certain way to help preserve the evidence. Items should be packaged in paper or cardboard, NOT plastic. Plastic containers and bags lead to an increase in moisture, which leads to the growth of mold, which interferes with the DNA process (see below).
One of the steps in forensic DNA analysis involves making copies of the DNA. Certain chemicals prevent, or inhibit, this process. Examples of inhibitors include humic acid (found in soil), mold, tannins (found in leather), indigo dyes (found in blue jeans), and melanin (found in hair).
Certain environmental factors can damage the DNA itself. Prolonged exposure to heat causes DNA to degrade (or break down). DNA is essentially a long tiny string. If it starts breaking into pieces, then it can’t be amplified for analysis. This idea applies both before and after the DNA is collected and packaged. If the evidence has been outside in the heat or high humidity before it’s collected or if it’s not stored at room temperature once it’s collected, the DNA can degrade.
Another environmental consideration is the elements themselves. A piece of evidence with blood on it that’s been outside in the rain would no longer be in the “High” category. The likelihood of getting a DNA profile would depend on how much blood is left, the type of item (small vs large, crevices where blood can hide, etc.), and where the item is found (maybe it was an area that was protected from moisture).
So, there you have it! Now you know potential sources of forensic DNA, which body fluids produce optimal results, and what can interfere with getting a good DNA profile. The great thing about writing your own crime fiction novel is that you get to decide how/where the DNA evidence is found and what the results are.
If you’re using forensic evidence in your crime fiction book and need someone to look it over for you for accuracy, head on over to my Services page. I provide professional beta reading, copyediting, and forensic consulting services, all of which include forensic science fact checking.