Accuracy
| Samples source | Positive rate |
|---|---|
| Bronchoalveolar lavage fluid specimens | 93% (14/15) |
| Sputum | 72% (75/104) |
| Nasal swabs | 63% (5/8) |
| Fibrobronchoscope brush biopsy | 46% (6/13) |
| Pharyngeal swabs | 32% (126/398) |
| Feces | 29% (44/153) |
| Blood | 1% (3/307) |
Accuracy is measured in terms of specificity and selectivity. Test errors can be false positives (the test is positive, but the virus is not present) or false negatives, (the test is negative, but the virus is present).
Sensitivity and specificity
Sensitivity indicates whether the test accurately identifies whether the virus is present. Each test requires a minimum level of viral load in order to produce a positive result. A 90% sensitive test will correctly identify 90% of infections, missing the other 10% (a false negative). Even relatively high sensitivity rates can produce high rates of false negatives in populations with low incidence rates.
Specificity indicates how well-targeted the test is to the virus in question. Highly specific tests pick up only the virus in question. Non-selective tests pick up other viruses as well. A 90% specific test will correctly identify 90% of those who are uninfected, leaving 10% with a false positive result.
Low-specificity tests have a low positive predictive value (PPV) when prevalence is low. For example, suppose incidence is 5%. Testing 100 people at random using a test that has a specificity of 95% would yield on average 5 people who are actually negative who would incorrectly test positive. Since 5% of the subjects actually are positive, another five would also test positive correctly, totaling 10 positive results. Thus, the PPV is 50%, an outcome no different from a coin toss. In this situation retesting those with a positive result increases the PPV to 94.5%, meaning that only 4.5% of the second tests would return the incorrect result, on average less than 1 incorrect result.
Causes of test error
Improper sample collection, exemplified by failure to acquire enough sample and failure to insert a swab deep into the nose. This results in insufficient viral load, one cause of low clinical sensitivity.
The time course of infection also affects accuracy. Samples may be collected before the virus has had a chance to establish itself or after the body has stopped its progress and begun to eliminate it. A May 2020 review of PCR-RT testing found that the median probability of a false-negative result decreased from 100% on day 1, to 67% on day 4. On the day of symptom onset, the probability was 38%, which decreased to 20% 3 days later.
Improper storage for too long a time can cause RNA breakdown and lead to wrong results as viral particles disintegrate.
Improper design and manufacture can yield inaccurate results. Millions of tests made in China were rejected by various countries throughout the period of March 2020 through May 2020.
Test makers typically report the accuracy levels of their tests when seeking approval from authorities. In some jurisdictions, these results are cross-validated by additional assessments. Reported results may not be achieved in clinical settings due to such operational inconsistencies.
PCR-based test
RT-PCR is the most accurate diagnostic test. It typically has high sensitivity and specificity in a laboratory setting: however, in one study sensitivity dropped to 66–88% clinically.
In one study sensitivity was highest at week one (100%), followed by 89.3%, 66.1%, 32.1%, 5.4% and zero by week six.
A Dutch CDC-led laboratory investigation compared 7 PCR kits. Test kits made by BGI, R-Biopharm AG, BGI, KH Medical and Seegene showed high sensitivity.
High sensitivity kits are recommended to assess people without symptoms, while lower sensitivity tests are adequate when diagnosing symptomatic patients.
The University of Oxford's Centre for Evidence-Based Medicine (CEBM) has pointed to mounting evidence that "a good proportion of 'new' mild cases and people re-testing positives via RT-PCR after quarantine or discharge from hospital are not infectious, but are simply clearing harmless virus particles which their immune system has efficiently dealt with" and have called for "an international effort to standardize and periodically calibrate testing" On 7 September, the UK government issued "guidance for procedures to be implemented in laboratories to provide assurance of positive SARS-CoV-2 RNA results during periods of low prevalence, when there is a reduction in the predictive value of positive test results."
Isothermal nucleic amplification test
One study reported that the ID Now COVID-19 test showed sensitivity of 85.2%. Abbott responded that the issue could have been caused by analysis delays. Another study rejected the test in their clinical setting because of this low sensitivity.
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