DNA
Testing Timeline
by: Heather R.
Fisher Sargent, MFS
Forensic Scientist
Human Identification Technologies, Inc.
As a Forensic DNA Analyst, I have often been asked why
DNA analysis takes so long. This is a rather
complex question because there are many contributing factors—the
huge DNA backlog, the insufficient
number of qualified DNA analysts in the laboratory, and
the constant influx of new cases. I have worked at
state, federal, and private crime laboratories and have
witnessed first-hand the severe backlogs that exist.
Although the throughput of DNA laboratories must increase
to meet demand, it is critical that the quality not
be sacrificed.
So, minus the backlog and related issues, how long does
it take to obtain DNA profiles from submitted
evidence? I can think of no better way to answer this
question than to take you step-by-step through what
needs to be done in a forensic DNA laboratory and approximately
how much time it takes to accomplish
these tasks. There are a few steps that take up time
throughout the analysis of a case including cleaning,
documentation, and quality control measures. It is difficult
to ascertain exactly how much time is devoted to
each of these tasks, which occur at each step of testing
from body fluid identification to the final DNA
typing. The following are routine tasks that take place
during and between the technical steps in DNA
analysis.
Cleaning
As forensic DNA analysts, it is incumbent upon
us to maintain a clean, contamination-free environment.
Every time new evidence is laid out and examined, the
surface of the examination table is cleaned with
bleach. Utensils such as scissors and tweezers are also
cleaned in bleach followed by ethanol and often
irradiated with ultraviolet light for several minutes.
The enclosed work areas known as hoods are
decontaminated with ultraviolet light. While cleaning
is second-nature to DNA analysts, the number of
minutes that it takes each day to clean may seem surprising.
Keeping hoods, utensils, and instruments clean
is an essential step in forensic DNA testing to prevent
contamination.
Documentation
Extensive note-taking and documentation
takes place at each step of the analysis process. It
begins from the time the evidence enters the laboratory,
which is documented via the chain of custody. Any movement
of
the evidence within the laboratory must be documented
to maintain this chain of custody. Communication
logs are often generated at laboratories to keep records
of various contacts with detectives, attorneys, etc.
When the analyst takes custody of evidence and performs
visual examinations, critical information about the
evidence must be documented. Evidence is also often photographed.
Documentation continues throughout
the DNA analysis from labeling tubes to meticulously
filling out forms at each phase of the process.
Quality Control Measures
There are several quality control
measures that are performed during the course of DNA
analysis. Various
instruments are used in the laboratory, which must be
maintained and calibrated to guarantee they are
working properly. Every time an analyst uses an instrument,
this information must be documented in a user
log. Temperatures of refrigerators and freezers that
store evidence and reagents are recorded daily.
Reagents necessary for DNA analysis must be documented
every time they are prepared with information
such as lot numbers and expiration dates. Lot numbers
of reagents used by the analysts are recorded
throughout the entire analysis in order to ensure traceability.
Technical
Steps Involved In DNA Analysis Of Casework
My goal here
is to give a reasonable time-line in hours of how long
it actually takes for a DNA analyst to
process evidence in a typical case, from body fluid identification
to a written report. To make things easy, I
am going to assume that I am given a relatively straightforward
case to analyze (versus a complicated
homicide case with numerous boxes of evidence). We will
assume throughout that I am processing a sexual assault
case with a rape kit, clothing items from the victim,
and reference samples from both the
victim and
suspect. Each technical step is described below with
time estimates in bold.
Body fluid screening of evidence
The initial process of
visually examining items of evidence for body fluid can
be quite time-consuming.
This basic step can actually be one of the most time
intensive aspects of analysis, depending on the type
of
case. This is also the most important aspect because
if biological material is not located, there will not
be any
DNA to analyze. At this point, forensic DNA analysts
are looking for any type of body fluid stains that
would be useful to the case, such as blood, semen, or
saliva. For example, in a rape kit there are several
swabs, including vaginal, anal, and oral that may be
analyzed. DNA analysts test these swabs for seminal
fluid and possibly perform microscopy to see if sperm
cells are present that would positively identify semen.
In other types of sexual assault cases with clothing
items, analysts use an alternate light source (ALS) to
visualize hidden stains and then further perform body
fluid identification. An ALS is commonly used to
visualize stains that cannot be seen by the naked eye,
such as seminal stains. Semen and other biological
stains can be made to fluoresce using the ALS with various
combinations of wavelengths and filters. It is
also important to keep in mind that forensic scientists
look for clues when screening evidence and must pay
attention to the fine details such as bloodstain patterns,
bloody fingerprints, and bloody shoe impressions.
Details of the case may demonstrate that certain bloodstains
are more probative than others. It is difficult to
give a time-line of this stage because it can vary.
Rape
kit: 2 hours 30 minutes of screening
Panties from victim: 2 hours of screening
Clothing from victim: 2 hours of screening
Total Time Spent
On Case At This Point: 6 hours 30 minutes
DNA Extractions
Once the stains have been located, forensic
DNA analysts focus attention on isolating the DNA from
these
stains. There are several types of extraction procedures
used in forensic laboratories today, some of which
are organic, organic differential, and QIAamp® DNA Micro extractions.
For sexual assault cases, such as
those including a rape kit with swabs, forensic DNA analysts
are interested in isolating both male (sperm)
and female (non-sperm) fractions, which is more time consuming
to do than an organic extraction of a
bloodstain, for example. The QIAamp® DNA Micro extraction is often
performed on reference samples
from victims and suspects. Reference samples are always
processed separately from questioned casework
samples to prevent any type of contamination. Reference
samples generally contain high amounts of DNA
relative to questioned casework samples. By processing
them separate from one another, the chance of
crossover contamination is eliminated. The amount of DNA
recovered from all samples in the extraction
procedure is documented in the forensic analyst’s notes, as well
as the amount of extract consumed for
analysis. In this way, the analyst knows how much sample
remains for possible re-analysis by another
laboratory.
Organic extraction: 4 hours 30 minutes
Organic differential extraction: 8 hours 30 minutes
QIAamp® DNA Micro extraction: 2 hours 30 minutes
Total Time Spent On
Case At This Point: 22 hours
DNA Quantitation
The next step is to determine how much
human DNA is present in the extracted samples. It is
important to know this amount so appropriate dilutions
can be made if there is a lot of DNA in the sample or the
extract can be concentrated if there is very little DNA
in the sample. The 310 Genetic Analyzer is one instrument
used to actually perform the DNA typing analysis, and
operates ideally when samples have a specific
concentration, which is why DNA analysts are so concerned
with knowing the amount of DNA present in the
extracted samples. DNA quantitation is performed using
a technique known as real-time PCR in most laboratories.
The analyst prepares the samples by adding chemicals
and then the samples are actually run
on
the instrument for approximately two hours.
Preparation
time: 1 hour 30 minutes
Samples running on instrument: 2 hours
Analysis of data by analyst: 45 minutes
Total Time Spent
On Case At This Point: 26 hours 15 minutes
PCR
Once the amount of human DNA in each sample is determined,
the necessary calculations are performed to
set up the PCR (Polymerase Chain Reaction) step. In the
PCR process, the DNA is copied or amplified and
tagged at physical locations on the DNA that are of interest
to forensic DNA analysts. Samples are diluted
and/or concentrated to meet the target amount of DNA
for the PCR reaction. The analyst prepares the
samples by adding chemicals and then they run on the
instrument for approximately 3 hours and 30 minutes.
Calculations: 30 minutes
Dilute/Concentrate DNA: 1 hour 30 minutes
Preparation of samples for PCR: 45 minutes
Samples running on instrument: 3 hours 30 minutes
Total
Time Spent On Case At This Point: 32 hours 30 minutes
DNA
Typing
The final technical step of the process involves
taking the copied and chemically tagged DNA from the
PCR
reaction and running it on an instrument such as the
310 Genetic Analyzer. This instrument separates the
chemically tagged DNA fragments by size, identifies them
through optics inside the instrument, and then by
computer software, generates data that analysts can review
and use to determine DNA profiles. The 310
Genetic Analyzer processes samples one at a time and
it takes approximately 35 minutes for each sample.
There are a number of control samples that are also run
in parallel with the casework DNA samples. For a
sexual assault case with questioned casework samples
(e.g. vaginal and anal swabs) and reference samples
from victim and suspect(s) there may be approximately
ten samples in addition to all of the control samples,
which brings the total to 25 samples being run on the
instrument. When the run is complete, the analyst must
interpret all of the data using complicated computer
software. There are numerous pages of data that need
to
be reviewed by the analyst so the DNA profiles can be
determined. After reviewing the data, statistical
analysis with the aid of a computer program is performed
by the analyst to determine how frequent the DNA
profiles are in the population. Then the analyst generates
a report summarizing all of the scientific findings
of the case.
Preparation of samples for 310 run: 45 minutes
Samples running on instrument: 12 hours 30 minutes
Analyst interprets/reviews data: 2 hours
Statistical analysis: 1 hour 30 minutes
Analyst generates a report/paginates case file: 1 hour
30 minutes
Total Time Spent On Case At This Point: 50
hours 45 minutes
Technical/Administrative Reviews
Once the analyst is finished
with the case file, it is handed to another qualified
forensic DNA analyst who
completes a thorough technical review of the case from
start to finish. The reviewing analyst evaluates the
case to make sure correct procedures were followed and
that the conclusions are based on proper scientific
analysis. After the technical review is complete, another
individual performs an administrative review, which
ensures that the report meets laboratory guidelines,
is grammatically correct and is free from typographical
errors and transpositions .
Technical review: 3 hours
Administrative review: 30 minutes
Total Time Spent on Case:
54 hours 15 minutes
Conclusion
Obviously, this time-line is a rough estimate
of how long it would take to analyze a relatively straightforward
case based on my experience in the laboratory. The total
time adds up to approximately 54 hours and 15
minutes. Along with the other tasks mentioned earlier—cleaning,
documentation, and quality control—it is
probably closer to 60 hours. As can be seen, the forensic
DNA analyst has many responsibilities, from
quality control to actual technical work in the laboratory
to technical reviews. There are other
responsibilities such as testifying in court, training
other laboratory personnel, etc. The analyst usually has
to
juggle multiple cases as well. There is no short-cut to
speed up DNA testing. With lives and liberty at stake,
the forensic DNA analyst has an obligation to produce
the highest quality of work and with this commitment
comes the cost of time.

As the largest private forensic DNA laboratory
in
Southern California, Human Identification
Technologies, Inc. (HIT) provides testing and
consulting services to our nation’s legal
community. Our single scientist case approach
from testing to testimony allows for consistent
communication between counsel and scientist
throughout the testing process. Through our DNA
testing and consulting services, HIT has assisted
in the resolution of numerous cases throughout
the nation.
Due to an extensive criminalistics background,
HIT’s forensic scientists have a unique
understanding of DNA’s role in the criminal
justice system and its impact in the courtroom.
This insight has resulted in a progressive
laboratory that is focused exclusively on the
efficient application of DNA technology in
criminal matters.
www.hitdna.com
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