What to know about at-home saliva tests
New Mexico’s rollout of home saliva tests for COVID-19 (“COVID-19 home test kits now available in New Mexico,” Dec 22) is undoubtedly a good thing — but only if New Mexicans know how to interpret the results. In particular, there is the risk that negative test results will create a false sense of security and lead people to go back to work or loosen precautions when they shouldn’t.
The saliva test is a polymerase chain reaction test, similar to the standard nasal swab test. The good news is the two tests appear to give similar results. But these results are not perfect. It is estimated that from 5 percent to 30 percent of negative nasal PCR tests are “false negatives.” In fact, sometimes people with COVID-19 have multiple negative swab tests before their diagnosis is finally made.
Vault Health, the company that sells the saliva test used by New Mexico, states the test has a false negative rate of only 1 percent. But this is from experiments comparing the saliva test against the nasal swab — an imperfect gold standard. To the extent the saliva test agrees with the nasal test (and agreement in these experiments was 100 percent), it will suffer from similar problems with false negatives when used in the real world.
The fact that the saliva test can easily be done at home is both its strength and its weakness. It means that more of these tests will be done without the benefit of a health care or public health professional’s interpretation of test results, in the context of the clinical situation and history of exposure.
We offer the following advice to people taking advantage of the home saliva test:
If your saliva test is negative: There is still a chance you have COVID-19 and can spread it to family members and others. If you are having symptoms of COVID19 or had close contact with someone with COVID-19, please stay in quarantine and repeat testing in a couple of days. The test is most likely to be accurate around five to seven days after exposure. (Note that Vault Health over-optimistically states that a negative test means you do not have the infection and cannot infect others — but does suggest considering repeat testing if you have symptoms.)
If your saliva test is inconclusive: A small number of saliva tests are reported as “inconclusive.” In this case, you should have another test, whether or not you have symptoms.
If your saliva test is positive: If the test is positive (and you have not had COVID-19 previously), it is very likely you have an active infection. There aren’t many “false positives” with the PCR swab tests (unlike with antibody testing). Quarantine and call your primary care physician.
If you have had COVID-19 previously: If you have had COVID-19, you may continue to have positive results at least three months after infection — long after you are no longer infectious. So it doesn’t make much sense to take a test within three months of a known infection (unless you are having active new symptoms that are strongly suggestive of reinfection and are advised to have a test by your physician).