Interpretation of COVID-19 RT PCR Positive & Negative Results on Sequential Samples
RT-PCR for SARS-CoV-2 is a diagnostic test designed for detecting the virus in respiratory samples of symptomatic individuals.
- It is therefore very specifc, so positive results are real sensitivity however is lower and false negative results may occur in up to 20% of swabs done, especially in mild or asymptomatically infected individuals.
- Referring doctors and patients usually have difficulty in interpreting these results, particularly when performed sequentially on patients, resulting in negative/positive or positive/negative discordances.
Reasons for these discordances include:
A positive result followed by a negative one:
- ”NICD” recommends that patients who test positive are treated as such and isolated and their contacts traced.
A low positive result (ct value37-40) may be followed by a second negative swab if:
- i. The patient is nearing the end of their infection and has stopped secreting virus
- ii. The second swab wasn’t taken from infected epithelium resulting in a false negative result
- iii. The patient is an intermittent secretor and was not secreting virus when the second swab was taken
- iv. The second result was obtained using a less sensitive assay than the first one