Polymerase Chain Reaction (PCR) for STI Detection and Testing

Polymerase chain reaction (PCR) analysis is a laboratory technique used to find small amounts of DNA (genetic material) in a sample. Among other applications, PCR can be used to detect multiple sexually transmitted infections (STIs). For example, a lab can find DNA in a urine sample that reveals gonorrhea or chlamydia.

PCR revolutionized the study of DNA and has been called one of the most important advances in molecular biology. It was first developed in the early 1980s by American biochemist Kary Mullis, who won the Nobel Prize in Chemistry in 1993.

PCR can be performed on DNA from many different types of samples, including:

• Urine
• Blood
• Sputum
• Stool
• Semen
• Cerebrospinal fluid
• Swabs of cells and fluids

How Does PCR Work?

The small bits of DNA in a sample are usually inadequate for analysis. PCR allows scientists to make a bunch of copies of the material (called amplification) quickly and inexpensively, which gives them enough to analyze.

Primers

The first step of the PCR process is to create what are called primers—short DNA sequences that can join up with the ends of the DNA sample you're trying to detect. They're the trick to finding, amplifying, and detecting a particular piece of DNA, which can then be used for things like:

• Identifying a pathogen
• Diagnosing genetic disorders
• Finding genes that influence antibiotic resistance
• Genome mapping

Separating the Strands

Once you have your primers, the next step in PCR is to heat the sample so that the double-stranded DNA separates into two single strands—this is called denaturation. Then the primers are combined with the sample DNA.

After this, a DNA polymerase (enzyme) is added and starts replicating the DNA where it meets up with the primer. The DNA then goes through the denaturating and replication procedure again and again.

Exponential Growth

With each cycle, the amount of the target DNA segment increases exponentially. In the first cycle, one copy becomes two. Then two copies become four, then become eight, etc.

Generally, it takes between 20 and 40 cycles to determine whether the target DNA is present. If so, by that time there's usually a sufficient sample for analysis, as well.

Automated Thermal Processing

All the steps of a polymerase chain reaction—denaturing the DNA, applying the primers, and elongating the DNA—happen at different temperatures. That means after the initial mixture is put together, the steps can be controlled through a process known as thermocycling.

Thermocycling means that the temperature is held at the necessary levels for just long enough for each step to take place. Thus, PCR is an efficient way of amplifying the amount of target DNA. In fact, it can be accomplished in a single test tube with little need for human intervention.

Why PCR Is Relevant to STI Testing

Polymerase chain reaction, and related techniques like ligase chain reaction, are growing in importance for STI testing. That's because these techniques can directly identify small amounts of viral DNA or RNA in samples.

Identifying the genetic code of a pathogen doesn't require the pathogen to be alive—unlike a bacterial culture or viral culture. It also means the infection can be recent enough that the body hasn't yet developed detectable antibodies for it, which gives it an advantage over a type of testing called ELISA.

Benefits of PCR

This means PCR techniques can sometimes detect sexually transmitted infections (and other diseases) earlier than other tests. Even better, samples don't need to be taken at exactly the right time, and they're easier to handle because you don't have to worry about keeping pathogens alive.

For medical facilities, these benefits mean faster, easier, less expensive tests. Tests are now available that can identify at least nine different STI-related pathogens at once, which makes them less labor-intensive, as well.

Home Testing

These benefits also have made home testing kits for STIs much more accurate and reliable. That's especially important for this kind of infection because a lot of people are embarrassed to go to their healthcare provider for testing, and home testing can often be done sooner than someone could get into see a healthcare provider, as well.

The combined benefit of more testing and earlier detection of STIs is a major step forward. Not only can prompt treatment help to prevent many complications, rapid identification means you can take immediate steps to keep from spreading it.

Types of STIs Detected by PCR

Medical facilities use PCR to detect a large number of pathogens related to sexually transmitted infections.

Bacterial STIs

Viral STIs

Parasitic STI

Frequently Asked Questions

How accurate are PCR test results?

Rates vary by disease and collection method, but PCR test results are highly accurate, according to medical studies. They fare well on both measures of accuracy:

• Sensitivity (the ability to identify the presence of a pathogen)
• Specificity (the ability to distinguish one pathogen from another)

What is multiplex PCR testing?

Multiplex PCR testing is when one test looks for multiple infectious agents simultaneously. Examples are STI PCR tests that look for up to nine pathogens.

What information is included in a PCR STI panel?

The results you get after a PCR STI panel, whether from a healthcare provider or self-test kit, should include information about:

• What viruses, bacteria, or parasites were tested for
• Whether your results are positive (you have an infection) or negative (you don't have an infection) for each pathogen

They may also have numbers representing the severity of an infection. Home test-kit results may offer further information about what to do if you did test positive for anything.

A Word From Verywell

STI testing is an important part of protecting your health, that of your partner(s), and possibly that of your future children. Just about everyone should get checked for STIs once, and depending on your age, overall health, and sexual practices, you may benefit from getting checked routinely.

Your healthcare provider can help you decide when and how often you should be screened for STIs. If you're not comfortable talking about it with your regular healthcare provider, you can go to a local clinic or look into home testing. However, remember that if you do have a positive test, you'll need to go to a healthcare provider for treatment.