Dr. Joseph Iser, MD, DrPH, MSc, FACPM is the former District Health Officer Southern Nevada and Washoe County Health Districts. This blog is adapted from a thread on his Twitter account @DrJosephIser.

We know accurate and widely available tests will be a critical component to being able to understand, get ahead of, and manage the COVID-19 outbreak. Lately, you may have heard a lot of discussion in the news about false positives and false negatives – let’s break these terms down and explain how they impact the success of a test.

Positive tests mean that the issue or condition you are being tested for, as ordered by your doctor, is present, according to the test. A negative is the opposite, what condition you are being tested for is not present. In the world of laboratory testing, positive is not good – for example, a positive TB test means you have tuberculosis, a positive COVID-19 test means you have the disease, etc.

A test coming back from the laboratory positive or negative is not the only thing to consider.
All tests have some margin of error – a very good test will have a very small margin, the results you receive from the test will in the vast majority of cases be in line with reality. Poor tests, like the initial tests distributed by CDC, have a large margin of error. Even though it may still be a small minority of cases – a significant number of those tested receive results that are not consistent with reality.

There are two major types of testing errors, false positives and false negatives. As the names suggest, a false positive test means that the laboratory results indicate you have a disease, while in reality you do not. A false negative means the test indicates you do not have a disease, but in fact you do. False test results are not just bad in the case of public health officials having inaccurate data for policy: a false positive test means your doctor will choose to treat you for a disease you do not have, which may result in medical complications. Likewise, a false negative means you will not receive treatment – or in the case of COVID-19, you may resume life without a period of isolation, infecting more individuals.

True positive and true negative results – where the test results match reality – are very good. They give public health officials an accurate picture of the disease’s spread, and they give front line health care workers the information they need to manage and treat patients.

A good test will have relatively few “false” results, and a good protocol accompanying the test will help to confirm the “true” results and weed out the false results.

There is one other thing to consider – interpretation of the results. Say we have a patient that is tested for COVID-19, and the test results are true – positive or negative – 100% of the time. The test might still not be useful if it is unable to tell us if a certain result means a person is able to, or unable to spread the disease.

Testing is a complicated endeavor, to beat this pandemic we are going to need accurate tests that provide useful information, and we are going to need enormous numbers of them.

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