Sensitivity of PCR tests in clinical practice: from cutoff values to false positives

In laboratory diagnostics of infectious diseases, PCR has become the gold standard. But even the most modern tests do not always guarantee an error-free result. The key factor determining their effectiveness is sensitivity, i.e. the ability to detect a minimal amount of genetic material of the pathogen. In this article, we will consider what high or low sensitivity means, how it is determined, why it can vary in real conditions, and how it affects clinical decisions.

Determination of sensitivity in PCR

The sensitivity of a PCR test refers to the smallest number of copies of a pathogen's DNA or RNA that the method can detect with high confidence. This value is measured in copies per microliter or in log₁₀ units (e.g., 10² copies/μL).

How is sensitivity determined: analytical and clinical approaches

In laboratory conditions analytical sensitivity is verified by serial dilution of the control material. For example, if the test detects SARS-CoV-2 in 95% cases at 200 RNA copies/µl, the LoD (limit of detection) is considered to be this number.

Clinical sensitivity — is the percentage of infected patients who test positive. Additional factors come into play here: the quality of sample collection, stage of the disease, and transportation conditions.

When high sensitivity is critical

• COVID-19 in the early stages — PCR allows you to detect infection before symptoms appear.

• HPV — Cervical testing for high-risk types of the virus with high sensitivity saves lives through early diagnosis of dysplasia.

• Herpesviruses in immunosuppressed patients — it is important to detect reactivation before clinical complications occur.

Pitfalls: why sensitivity decreases in practice

• Improper specimen collection: Nasopharyngeal swabs may not contain enough cells.

• Sampling too late: 7–10 days after infection, the amount of RNA in the respiratory tract decreases.

• Freezing/thawing the sample may degrade nucleic acids.

Hypersensitivity: always good?

In some cases, the test may detect residual or clinically insignificant amounts of the virus. For example, a PCR for COVID-19 may be positive even after a full recovery - these are traces of non-infectious genetic material. This creates false impression of chronic infection.

Alternatives and comparisons

PCR is superior to immunological methods in the incubation phase of infection. However, for definitive confirmation, some protocols (e.g. WHO in tuberculosis) involve additional microbiological testing or serology.

Practical recommendations for the doctor

1. Explain to patients that a negative PCR does not always rule out the disease.

2. In case of high clinical risk, recommend retesting in 24–48 hours.

3. If you have a positive result after recovery, evaluate the clinic, not just the laboratory indicators.

4. Always check the quality certificate of the test system: LoD, validation, ECDC or CDC recommendations.