How Long Do Forensic DNA Results Take?

by | Jan 10, 2022

Are you using forensic DNA in your crime fiction book, but aren’t sure how long it takes to get the results back from the crime lab?

On TV, the characters get the forensic DNA results back in an hour. You’ve also heard about backlogs in the news where the results take months, if not years. You know the former is wrong, but the second scenario doesn’t work for your book.

What’s a crime fiction writer to do?

The simple answer: it depends.

Many factors can affect how long a DNA case takes.

In my 7-year career as a forensic scientist, the turnaround time for cases in the DNA section fluctuated from 14 months (when I first started) to 2 months (about halfway through my time there) to 6 to 8 months (when I left). The decrease in turnaround time was the result of a new technology/workflow and two additional forensic scientist positions. It increased because of an influx in submissions and staff turnover.

In this post, I explain the different factors that can affect turnaround time and why some cases take longer than others. You can use these different scenarios as plot points in your book to make your forensic DNA results more realistic.

But first, we need to go over the forensic DNA analysis workflow.

Overview of Forensic DNA Analysis in a Crime Lab

1. Screening

A forensic DNA analyst first looks for body fluids and other biological materials on the evidence. If the case is positive, representative samples are selected for DNA testing. If the case is negative, a report is written and technically reviewed, and the evidence is sent back to the agency.

2. DNA Analysis

Samples selected for DNA testing are taken through a very specific workflow that includes getting the DNA out of the cells, figuring out how much DNA is present, making copies so there’s enough DNA to work with, and running the amplified DNA on an instrument called a genetic analyzer to get the DNA profile.

3. Data Interpretation

The DNA profiles from all the samples in a case are analyzed by a software program and interpreted by the DNA analyst. The analyst then compares the profiles and makes conclusions.

4. Report Writing

Once the conclusions are made, the DNA analyst writes a report on his or her findings.

5. Technical Review

In forensic DNA casework, all reports must be technically reviewed by another qualified DNA analyst. The reviewer goes through the case file and reviews the findings and conclusions for technical accuracy. Once the reviewer signs off on the report, the evidence is released back to the agency and the report is sent out.

How long it takes a case to go through this process can vary. The exact workflow can differ from lab to lab, but DNA cases are typically taken through the above process in batches (which is more efficient).

Factors That Affect Forensic DNA Turnaround Time

Type of Analysis

There are multiple types of forensic DNA testing. Nuclear DNA testing (the most common) is faster than mitochondrial DNA and investigative genetic genealogy testing.

Public vs Private Lab

Public crime labs provide forensic testing services to agencies for free and are funded by taxpayers. However, many public crime labs have backlogs. Private labs provide forensic DNA testing services for a fee. Testing at private labs is faster. 

Type of Case

Another factor is the type of case. The screening process for sexual assaults takes longer because the tests used to screen for the presence of semen are more time-consuming. The DNA portion of the workflow can also take longer because getting the DNA out of sperm cells requires a special (longer) procedure.

Number of Items

The size of the case (i.e., number of items submitted for analysis) affects how long forensic DNA results take. A complicated homicide or assault case with dozens of items and multiple suspects and victims will take a lot longer than a simple burglary with a few items.

Additional Testing

After analyzing the data, the analyst may decide to go back and take another round of samples through the workflow or re-run samples.

In a larger case, the analyst may go back and choose a different set of stains to ensure a representative sample was analyzed.

In addition, positive and negative controls are used at every stage in the entire process to ensure that quality results are obtained. If the controls fail or do not work properly, the samples associated with those controls need to be re-run.

Difficult Samples/Troubleshooting

Troubleshooting instruments can also interfere with getting timely results.

Not only is the instrument out of commission, which can interfere with productivity, but someone has to troubleshoot with tech support to figure out the problem, which means that analyst isn’t doing casework.

I once had to troubleshoot a genetic analyzer for 3 weeks! Other cases were being worked by other analysts, and I was able to tech review cases in between troubleshooting tasks, but I was not working new cases during that time.

Difficult samples or unexpected results can also be a reason for re-analysis. I once worked a homicide case where I took cuttings from a bloodstain on a pair of jeans. When I analyzed the data, there was no DNA profile. I went back and took another cutting, and still no profile. I had other DNA results in the case, so I didn’t go back a third time, but the re-analysis extended the turnaround time for that particular case.

Availability of Crime Lab Personnel

Availability of crime lab personnel can have a major effect on turnaround time.

People go on vacation, attend trainings/conferences, get sick, have medical/family emergencies, go on leave, and so on.

Lack of personnel hits a smaller lab (like the one I worked at) especially hard. If someone resigns, the position isn’t filled right away (it can take months), and when it eventually is, someone has to train the new person, taking the trainer away from casework.

Forensic scientists have other duties in addition to casework, such as meetings, court, presentations, instrument maintenance, audits, and trainings.

Waiting on Submitter

Cases were also delayed if we were waiting for information from the submitting agency or prosecuting attorney. We would often have to call and ask questions prior to completing our analysis. If they didn’t respond in a timely manner, the case would sit until they did.

What Turnaround Time to Use for DNA Analysis in Your Books

Now that you know the different factors that can affect forensic DNA testing turnaround time, what timeframe should you use in your fiction books?

As a general rule, I advise crime fiction authors to use a minimum turnaround time of 2 weeks for nuclear DNA testing at a public crime lab. It’s faster than months/years, but is more realistic than an hour.

You can also add in different scenarios/tweaks based on the different factors I mentioned above to get your results faster. You can send the items to a private lab or only test one or two items.

If you’d like more guidance on how to use forensic DNA in your fiction books (and get the results back even faster), be sure to check out Write Crime Right: Forensic DNA Fundamentals, my beginner course for for fiction writers. In the course, we build the forensic DNA framework for your book, figure out which forensic DNA technology is best suited to the DNA evidence you want to use, and find the best and most believable timeframe for forensic DNA testing.



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