All you wanted to know about test positivity rate

Kumar Shankar Roy | Updated on May 19, 2021

The word ‘positive’ has a nice ring to it, but not these days when it is used in conjunction with the much-dreaded Covid test. The test positivity rate or TPR is an important metric through which the public health system keeps tabs on the current level of Covid transmission.

What is it?

The TPR is the percentage of all coronavirus tests performed that turn out to be positive, and detect the presence of the virus. The formula used is to divide the number of positive tests (numerator) by the total number of tests done (denominator) and multiply this result by 100. The total here equals positive plus negative test results and excludes undetermined results. According to criteria published by WHO in May 2020, a TPR of less than 5 per cent is one indicator that the infection is under control in a locality.

Why is it important?

TPR is the key metric being used to assess whether the Covid spread is getting arrested in a geographical area. For instance, India's overall virus positivity rate fell to 19.8 per cent last week from 21.9 per cent in the preceding period. Thanks to containment measures and localised lockdowns, the situation has slightly improved because even a few days back the positivity rate was 20 per cent or more in two out of every five districts in India.

But do note that an extremely high positivity rate, for instance the 91.5 per cent in Changlang district of Arunachal Pradesh (91.5 per cent) is possibly an indication of an extremely low number of tests conducted rather than a high number of infections. As per norms, daily testing has to be raised in areas with high test positivity rates. When data is used appropriately, it can also throw light on whether only a subset of the community is at greatest risk for infection.

So, what’s a high TPR? The interpretation depends on the volume of Covid diagnostic laboratory testing done as well as the populations that are being tested. For example, the numbers can be different when there is routine screening of asymptomatic persons versus diagnostic testing of symptomatic persons or contacts of Covid patients. Therefore, we must bear in mind that TPR rates very much depend on who is being tested. For instance, a low TPR is not good news if the testing has been expanded to more people who are not infected (the denominator is larger without uncovering a lot of new infections). Testing high-risk people is more important.

Why should I care?

Because testing kits are also in short supply and diagnostic labs are over-worked, limited or slow testing makes it likely that the number of infections is under-reported. TPR can help understand the spread of the virus so adequate measures can be taken to control it. For instance, in locations with a high rate, the number of confirmed cases is likely to represent only a small fraction of the true number of infections. Hence, containment measures will be critical in controlling the spread.

If the positivity rate is rising in a location, this can suggest the virus is actually spreading faster than the growth seen in confirmed cases. A new variant or variants can cause much faster transmission. So, attention has to be given to such a situation, even in the absence of immediate medical proof. While positivity rate makes for good headlines, there is peril in basing our decisions on just one ratio. TPR and its movement should ideally be seen in conjunction with the number of positive cases, tests, mortalities, hospitalisations and their respective movement.

The bottomline

Making people's immunity resilient through vaccination is key. The virus will continue infecting people whether we detect more cases or not.

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