What you need to know
- The positive predictive value of a positive lateral flow device (LFD) test depends on the underlying likelihood of disease
- When the disease incidence is low, a positive result should be validated by a polymerase chain reaction (PCR) test. However, if your clinical opinion is that covid-19 is likely, then a positive test is likely to be reliable
- LFD testing is not recommended when the person has symptoms of covid-19, as a negative LFD is not sufficient to rule out covid-19
- If a symptomatic patient informs you that they have had a negative covid-19 test, check what type of test was done
- If covid-19 is clinically suspected, a PCR test is recommended, even if the patient has received a negative result from a recent LFD test
Then question is can we in Sri Lanka without knowing the TRUE disease incidence use the Rapid Antigen Test accurately?
Use the direct link to alter the Pre-Test probability of this Infographic – LINK
What do clinicians need to know to understand a test result?
Test characteristics (sensitivity and specificity) alone are of limited value in interpreting the test result. Knowing the pre-test probability, or the underlying likelihood of an individual having covid-19, is vital for interpreting the test result.
To assess this, inquire about why the test was done, as well as other factors that might influence underlying risk of covid-19, including:
• epidemiological link (eg, contact with a known case or link to an outbreak)
• travel to or residence in an area of higher transmission • occupational risk
• symptoms suggestive of covid-19
• vaccination status
• history of previous infection.
A good understanding of the local epidemiology (local UK data are available at https://coronavirus.data.gov.uk/) can improve interpretation. Where it is known, it may be helpful to shift the pre-test probability up or down based on age or other risk factors. For example, if assessing the result of a student, knowledge of recent outbreaks among students or high infection rates in young adults, would push an estimate up. Conversely, rates of infection tend to be lower in older adults who have fewer social contacts and (in the UK) are now mostly immunised.
Also consider the quality of the testing (eg, who did the test, their familiarity with testing, and whether they used a recognised test). The sensitivity and possibly the specificity may decline if the quality of testing is weaker.