Science, experience, and learnings pertaining to the COVID19 test is evolving in the world as well as at Metropolis. Following is some useful information that is being shared to overcome prevailing myths:
How reliable is a NEGATIVE result?
While RT PCR is the best testing platform available as of now, there are good enough chances of getting false-negative result due to the following: –
The test done from nasopharyngeal swab gives as high as 44% false negative.
The viral load may vary from sample to sample and hence occasional sample may have too low viral count to detect
Testing kits sensitivity not thoroughly verified
Improperly collected nasopharyngeal swab can give a false negative result
Viral load continues to go down and may get undetected after the 7th day. 2, 3
Viral RNA is labile and may deteriorate if the sample is not transported/stored properly
How reliable is a POSITIVE result?
Positive results are fairly reliable based on the following criteria: –
The test involves screening as well as a confirmatory test before the patient is declared “POSITIVE”. This itself is a double check
Multiple genes are detected for a patient to be labeled “POSITIVE”
Data on False positivity is not available in the current published literature. Also, data on the comparability of different kits still not available.
Under what circumstances, a patient tested POSITIVE at one lab would give NEGATIVE on repeat?
There are multiple scenarios of repeat testing:
If the Same sample is repeated, a negative result may come due to poor sensitivity of alternate kit used, different genes used or there is sample deterioration. However, one would expect a lesser chance of this happening
If the new sample is taken on the same day, a negative result may come due to improper sample collection, poor sensitivity of alternate kit used, a different gene used or there is sample deterioration. However, chances of change are also comparatively low
If the new sample is taken a few days apart, chances of the changed results are very high because the viral load reduces day by day. The patients having better immunity starts clearing the virus faster and may test negative sooner. The study shows that virus can get cleared as early as 4.3 days of infection or getting the first test positive
Under what circumstances, a patient tested NEGATIVE at one lab would give POSITIVE with repeat sample collected?
If a patient gets tested himself too early (< 2 days of infection), there is a chance of the first test being false negative. On repeat, he may get positive findings
Improperly collected sample gives false-negative and hence such patient going for repeat test may turn positive
The test involves the detection of viral RNA, which is extremely labile. If the first sample is not properly stored, it will give a false negative. Hence, a repeat test may come positive.
If a different type of sample is used for the test, one may get subsequent test positive. E.g. First time nasopharyngeal swab is taken but the second time sputum or saliva is taken.
What is ideal time for COVID19 – RT PCR test?
As soon as one gets the symptoms. Viral shedding is maximum just before symptoms appear.4
On day 5 in asymptomatic and high-risk contacts (as per ICMR 5 )
State rules to be followed for accepting patients for testing
How does one know that he/she is free from the virus?
As of now, the commonly used method of proving viral clearance is to get two RT PCR tests done consecutively. If both come negative, viral clearance is declared. However, there are exceptions cited in the literature, and the patient may still shed virus in stool for 4 to 5 weeks after infection. 6
What are the pre-examination factors that can give false results?
Incorrect timing of the test after infection.
Incorrect choice of testing method.
Incorrect sample type that gives low sensitivity.
Poor sample collection
Inadequate sample storage.
Can an asymptomatic person get COVID19 test positive?
Yes. But as per current (as on 22nd April) BMC guidelines, only symptomatic patients are to be tested with some exceptions (pregnant women 34 weeks onwards, patient on chemotherapy and dialysis). 7 ICMR has shared that up to 69% of patients are asymptomatic in the data analyzed by them
References:
Yang et al. Evaluating the accuracy of different respiratory specimens in the laboratory diagnosis and monitoring the viral shedding of 2019-nCoV infections.
Correlation of Chest CT and RT-PCR Testing in Coronavirus Disease 2019 (COVID-19) in China: A Report of 1014 Cases
Kelvin et al. Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study
Carl et al. SARS COV 2 viral load and severity of COVID-19
ICMR revised the testing strategy for Covid19 in India, 20 March 2020.
Yongjian et al. Prolonged presence of SARS-CoV-2 viral RNA in fecal samples
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