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Jan 22, 2016Print this page

Swabbing for Trace Evidence vs. Swabbing for Blood and Other Fluids


Swabbing for blood or other fluids and collecting trace evidence are two totally different swab games.

Collecting fluid almost always involves the use of a swab applied to the visible portion of a piece of evidence. But collecting trace evidence isn't always as straightforward.  

While some methods of trace evidence collection might involve the careful use of sterilized needle-nose tweezers, tape, and a dry transport system (or envelope), other types of trace evidence aren’t visible to the naked eye.

That’s when your trusty swab comes in so handy.

To collect touch DNA from fingerprints or other skin cells, or residue from gunshots, explosions, or arson, a swab is often the best tool for the job.

But how is collecting trace evidence with a swab different from using a swab to collect blood, semen, or other fluids?

It comes down to two important factors: the type of swab and the method of collection. Check out our previous post about how to swab for DNA evidence at crime scenes for a breakdown of the specific collection methods for each type of evidence. 

Here, we discuss how to choose the right swab for the job. 

Choose DNA-Controlled Swabs

When you're swabbing for evidence at crime scenes, you've got options. But where DNA is concerned, it's best to choose a swab that's specifically labeled DNA-free or DNA-controlled. This means that they are free of any human contaminants like DNA, DNase, and RNase; which could compromise a sample—and ultimately, a case.

Puritan's DNA-free and RNase-free sampling swabs have been certified by an independent laboratory to be DNA-free at a threshold lower than any other swabs in the market today.

Swabbing Fluids like Blood, Semen, Saliva and Sweat

Flocked Swabs

If you're dealing with an extremely small sample size and/or you can afford a superior alternative to your standard cotton swab, we recommend choosing a DNA-controlled flocked swab

Flocked swabs are becoming increasingly popular in forensic circles because of their unique micro structure which gives them the ability to collect, retain, and elute specimen better than other types of swabs. Because of this, flocked swabs are ideal for PCR, molecular assays, DNA testing, and direct antigen testing.

Cotton Swabs

If you have a large enough sample and you don't have advanced collection and elution needs, a sterile and DNA-controlled cotton tipped swab with either a wooden or polystyrene handle is acceptable. 

You'll Need a Comparison DNA Sample

DNA collected at a crime scene is useless without a comparison sample. In addition to the ultimate goal (identifying a suspect), comparison samples are also used to eliminate suspects and/or others who may have been on the premises prior to a crime being committed. 

Most often, buccal cells—taken from inside the cheek—are ideal for this type of analysis. To collect buccal cells, we recommend either foam tipped swabs, or cotton tipped swabs; depending on both your methods of analysis and your concern for patient discomfort. 

Foam Swabs

If you need to transfer your sample to FTA cards, you'll want to choose a large, round foam-tipped swab. The extra large tip of this type of swab provides extra surface area for maximum sample collection and preservation. Foam tipped swabs have the added benefit of being soft to the touch—minimizing discomfort. 

Cotton Swabs

If you do not need to transfer your sample to FTA cards, and patient discomfort isn't a major concern, we recommend a sterile medical grade cotton swab with a sturdy handle. Ideally, you'd choose a product that includes its own dry transport tube to eliminate the possibility of contamination during transport.

Puritan's forensic cotton swabs come with a reclosable, dry transport tube with a patented design and breathable filter label that covers holes to reduce the risk of bacterial overgrowth during transport.

Swabbing Trace Evidence like Touch DNA and Gunshot Residue 

When you're swabbing for trace evidence it's important to be mindful of both the sample size and sample type. Click here for explicit instructions on how to swab for touch DNA (versus other types of trace evidence). You'll find details on exactly what touch DNA is, how its processed, and how best to collect samples while avoiding contamination.

Flocked Swabs

For small samples, you'll want the precision and absorption capabilities offered by flocked swabs. 

Self-Saturating Foam Swabs

For larger samples, a foam-tipped swab is fine. To minimize the risk of contamination from other tools (like your pipette) the best swab for the job is a self-saturating one. 

DNA Evidence Collection Flowchart


Topics: Forensics


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