What the Covid19 test yield tells us – Are we undercounting?

Dr Richard Ayah

Compared to the last 6 days when 300-600+ cases per day were being announced, fewer covid19 positive cases were announced today – 195 from 1,852 tests. A total of 1,084 patients are currently admitted, and in recent press briefings the Ministry of Health has started to give more information, a sign of improving communication. Seven more deaths were recorded in the last 24 hours the crude fatality rate now 1.86%.

The tenor of the Ministry of Health is that there is a second wave if Kenyans do not practice covid19 prevention measures. However, the question is whether the cases went down and tests are picking up a surge or there was limited testing and increased testing is picking cases that were ongoing.

7 Day Running Average Yield and 7 Day Average tests done 
8th April – 18th October 2020 (Adapted from Ministry of Health data)

In the chart above the orange line shows the 7 day average yield (we take an 7 day average to try smooth out the daily fluctuations in testing). In early April 2020, the initial testing was of person quarantined returning from flights above. We did not yet have community transmission. As the test yield declined it become evident that the group still in quarantine no longer had covid among them. A University of Nairobi research blog article on April 12th  wondered if Kenya was now entering a new phase. Hindsight shows that indeed community transmission had started and as the number of tests done rose so did the number of positive cases, with peaks and troughs closely following each other.

The month of September 2020, so a dip in the number of positive cases, again in line with the drop in testing and a corresponding drop in the yield. At first glance this might appear contradictory, shouldn’t the yield rise if the cases are still there but testing is reduced (reducing the denominator but not the numerator). This is true so far as the criteria for testing remains the same. In the case of Kenya, once government slowed down testing, essentially the tests being done were those who could afford tests – say travellers boarding flights or business organisations where there has been a ‘corona scare’. Many patients in hospitals who might otherwise have been tested were not – perhaps because insurance is not paying for testing and care; but also because the test was simply not available. Without any index cases, no contact tracing then takes place and so those in contact with someone who likely has it were not tested (remember 93% of covid19 positive cases are asymptomatic).

In October there has been an increase in testing especially in public hospitals and now there is an increase again in the covid19 cases. It looks like June/July repeating on our way to August but it may also be that September was an anomaly because of low testing and that a weak health system missed to count the cases.

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Dr Richard Ayah

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