Intended for healthcare professionals

Rapid response to:

Feature Briefing

Covid-19: How the UK is using lateral flow tests in the pandemic

BMJ 2021; 372 doi: (Published 04 February 2021) Cite this as: BMJ 2021;372:n287

Rapid Response:

PCR testing for SARS-CoV-2 is far from 100% sensitive

Dear Editor

I have increasingly noticed publications comparing tests to PCR tests in ways that imply that PCR is the "gold standard" for SARS-CoV-2 RNA.

While it is acknowledged that PCR testing is exquisitely specific (there are very few false positives, and when they occur it's usually do to contamination of the sample), and it is exquisitely sensitive when there is viral RNA on the swab, we also know that in the real world, many patients who have genuine Covid-19 disease (based on subsequent confirmation and/or classic constellation of typical signs and symptoms) test negative on PCR testing of swabs. Even in hospital - it is not entirely down to poor technique when performed eg by the patients or poorly-trained volunteers at test centres.

Sensitivity of PCR testing in the real world (taking into consideration the likelihood of the virus getting onto the swab) is only of the order of 70%. It misses (gives false negative results) in about 30% of people who do actually have the virus, and who may be contagious.

So, comparing, say, lateral flow or LAMP tests with PCR, assuming that a negative PCR test means the other test is definitely a true negative, fails to take into consideration the false negative rate of PCR tests, rendering them even less useful.

Competing interests: No competing interests

04 February 2021
Peter Mark English
Public Health Physician
N/A - responding in a personal capacity