The Stability of Test Positivity In India

19 May 2020
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Figure 1: Test Positivity for Covid-19 in India over time (this chart as of  July 09 2020)

(Source: ICMR and Ministry of Health and Family Welfare)

This graph shows the best possible estimate of test positivity from available official data.

As of July 9th, 2020, test positivity is 7.1% and the incremental test positivity is 9.3%. 

Test positivity in this case is the number of persons testing positive divided by the number of samples tested.

`ICMR is currently testing over 90 thousand samples a day, a huge increase from one thousand a day in mid-March. This does not mean persons – because some of these samples are from people already tested positive who need to be tested again to see if they have recovered or whether they are still positive.

Ideally, the test positivity would be the number of persons testing positive divided by the number of persons tested, but this data is not released by official sources (it used to be).  Table 1 below shows the kind of information related by ICMR at different times. Between 9 to 19 April, ICMR released the most information,  number of total samples tested, the number of total people tested, the number of persons found positive, but since 25th April, they have only released the total number of samples tested. The MOH&FW used to also release the data on number of positive persons, which did not always match with ICMR. Since 25th April, only MOH&FW releases this data. The MOH&FW website continues to say their “figures are being reconciled with ICMR”.

The ICMR data can have shortcomings, e.g. there may be mis-identification of the location, duplicate records due to the multiple samples per patient, non-use of the unique ICMR ID for subsequent tests (see the explanation given by Brihanmumbai Municipal Corporation), but it is telling that reconciliation is proving extremely difficult and even after more than three weeks, it has not been possible to reconcile the data.

Source: https://api.covid19india.org/csv/latest/statewise_tested_numbers_data.csv

Test positivity is trending upward in almost all states

The updated Figure 2a shows that test positivity has moved up in almost all states – which would be the case, if the national test positivity is steadily trending upward. Within these, some states, notably Madhya Pradesh, West Bengal, Chhattisgarh and especially Punjab, the test positivity is trending down.  The improved explanatory power indicates that deviations from the broad pattern of May 10 are lesser over the past month than the churn between April and May.

States are expanding testing at a similar pace

Compared to the earlier graph, Figure 3, we see in Figure 3a that the increase in testing between May 10 and June 10 has been much more proportional across states, with most states clustering on the 45 degree line and both the coefficient getting closer to 1 and the explanatory power of the regression increasing, indicating that deviations from the trend are not large.  This is a welcome trend, subject to qualifications below.

Discourses that excoriate states for not testing enough miss this trend.  Yes, some states needed to catch up, from their initial deficit in testing capacity, but by 10 May, there was a stable structure that seems to have been put in place.

Maharashtra, Delhi, Bihar, Gujarat and Tamil Nadu perhaps need to be testing more, given their rise in test positivity but we see that their share of testing has either remained same or has gone down. That would need attention and importantly, support – so that unused testing capacity in other states can be put to use.  West Bengal, on the other hand, has both increased its share of testing and shown a reduced positivity, indicating that they appear to have managed the infection spread from returning residents thus far. 

What does it mean if the test positivity remains constant, as the number of daily tests are increased almost 100X?

One would expect that as more people are tested the share of those infected would fall, but that has proved not to be the case at the national scale, though in many states, as seen in Figure 2, there has been a decline in test positivity over time. All the states below the 45 degree line show a lower test positivity on May 10, than on April 10, reflecting a decline as testing is increased. Gujarat and Maharashtra show a contrary trend and since they have a large number of cases, their contribution to the national ratio is high.

The constancy in the national test positivity masks significant churn within the states. This data is not released officially by the MOH&FW, but is compiled by crowdsourcing platforms from various official announcements. This data shows significant variation across districts and over time. As seen in Figure 2, there is considerable variation across states in test positivity. Over time, as testing as increased, the test positivity of most states has declined with the notable exception of Maharashtra and Gujarat.  However, the crowd sourced data does not match with the data released by MoH&FW. Indeed, the test positivity is a little lower in the crowd sourced data than in the official release. There is also a lot of change in the testing being carried out across states as seen in Figure 3. It is important to put in reliable process to release data at state and district level if a sensible understanding of the crisis is to be attempted.

But, it is better news than a rising test positivity. Many of those who are being tested are likely to be infected because they were high risk contacts of people who tested positive for the virus. So, one would expect that the population prevalence to be much lower than that of the tested population. After the core group of contacts of people who have tested positive – both symptomatic and asymptomatic – there are other groups like SARI patients and symptomatic people with ILI in hotspots. But, a large number of patients are asymptomatic. Dr Gangakhedkar, ICMR’s head of epidemiology and communicable diseases said thatif we look at the number of tests done, so far 31% belong to the symptomatic category and the rest 69% would fall under asymptomatic.” Similarly, in Maharashtra, which releases these figures daily, over 70% of the cases are asymptomatic. Are all these people asymptomatic contacts? If not, how did they come to be tested? Till we know who is being tested, in comparison to the population, it is difficult to answer the question as to what this test positivity tells us about infection in the general population more precisely. The experience of pilgrims who returned from Nanded to Punjab, where about 30% tested positive, but were asymptomatic, indicates that there is much to know still about its transmission and virulence. But, beyond broad generalities, there is limited information on who is being tested.

Source: https://api.covid19india.org/csv/latest/statewise_tested_numbers_data.csv

ThePrint India carried this analysis by Partha Mukhopadhyay. Access the piece here.

Note:

The data on positive persons is released by the Ministry of Health and Family Welfare (MOH&FW) at 8AM every day.

The number of samples tested is released by the ICMR at 9AM

The figure will be updated on a daily basis. The associated data is available here.

 

The views shared belong to individual faculty and researchers and do not represent an institutional stance on the issue.