How many days after exposure should one be tested to yield the most accurate results, and with which test?
How many days after exposure should one be tested to yield the most accurate results, and with which test?
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Research suggests that diagnostic testing is more accurate a few days after symptoms start, or around a week after exposure to a person who is infected with COVID-19. Testing more than once can confirm negative results, when appropriate, and when tests are available. During the wait for test results, it is essential for people who suspect they have COVID-19 or have been exposed to COVID-19, to take precautions and self-isolate when possible. It is also important to consider what type of test is being used to check for a COVID-19 infection, as this will likely impact how accurate the test is and how long it will take to get results. Molecular tests are among the most accurate diagnostic tests currently available for detecting whether someone has an active COVID-19 infection. Molecular tests use methods such as RT-PCR (reverse transcription polymerase chain reaction) to detect genetic material from SARS-CoV-2, the virus that causes COVID-19, in respiratory samples such as nose and throat swabs. Molecular tests have a higher risk of false negatives in the earliest days after exposure and symptom onset, according to an August 2020 publication in the Annals of Internal Medicine by researchers at John Hopkins University who reviewed 7 published studies on the performance of RT-PCR molecular tests. The researchers found that on average, the false negative rate was lowest around day 8 of an infection or 3 days after symptom onset (symptom onset is typically several days after an infection starts), with the false negative rate rising again as the infection continues. False negative test results in the early stages of infection are concerning, because other research (including studies published in Nature and the American Journal of Pathology) have found that COVID-19 patients can be most infectious to others in the early days of infection, when test results may be more likely to come back as false negatives. Some testing policies recommend that people get tested twice to confirm a negative result. Repeat testing to confirm negative results can be particularly important for people who may interact with high-risk populations (ex. healthcare workers, caretakers), people who may interact with many others outside of their household (ex. an employee going back to the office, a student returning to in-person classes), and people who may need medical care for COVID-19 (e.g. elderly patients with underlying conditions). Molecular tests are a relatively accurate type of diagnostic testing, and they have a lower chance of false negatives when conducted a few days after symptoms start, or approximately a week after exposure. A lower chance of false negatives does not mean there is no chance of inaccurate test results, so repeat testing may be recommended to confirm test results in certain situations. With all the ongoing research and development work on COVID-19 tests, pandemic testing guidelines may continue to evolve.
Research suggests that diagnostic testing is more accurate a few days after symptoms start, or around a week after exposure to a person who is infected with COVID-19. Testing more than once can confirm negative results, when appropriate, and when tests are available. During the wait for test results, it is essential for people who suspect they have COVID-19 or have been exposed to COVID-19, to take precautions and self-isolate when possible. It is also important to consider what type of test is being used to check for a COVID-19 infection, as this will likely impact how accurate the test is and how long it will take to get results. Molecular tests are among the most accurate diagnostic tests currently available for detecting whether someone has an active COVID-19 infection. Molecular tests use methods such as RT-PCR (reverse transcription polymerase chain reaction) to detect genetic material from SARS-CoV-2, the virus that causes COVID-19, in respiratory samples such as nose and throat swabs. Molecular tests have a higher risk of false negatives in the earliest days after exposure and symptom onset, according to an August 2020 publication in the Annals of Internal Medicine by researchers at John Hopkins University who reviewed 7 published studies on the performance of RT-PCR molecular tests. The researchers found that on average, the false negative rate was lowest around day 8 of an infection or 3 days after symptom onset (symptom onset is typically several days after an infection starts), with the false negative rate rising again as the infection continues. False negative test results in the early stages of infection are concerning, because other research (including studies published in Nature and the American Journal of Pathology) have found that COVID-19 patients can be most infectious to others in the early days of infection, when test results may be more likely to come back as false negatives. Some testing policies recommend that people get tested twice to confirm a negative result. Repeat testing to confirm negative results can be particularly important for people who may interact with high-risk populations (ex. healthcare workers, caretakers), people who may interact with many others outside of their household (ex. an employee going back to the office, a student returning to in-person classes), and people who may need medical care for COVID-19 (e.g. elderly patients with underlying conditions). Molecular tests are a relatively accurate type of diagnostic testing, and they have a lower chance of false negatives when conducted a few days after symptoms start, or approximately a week after exposure. A lower chance of false negatives does not mean there is no chance of inaccurate test results, so repeat testing may be recommended to confirm test results in certain situations. With all the ongoing research and development work on COVID-19 tests, pandemic testing guidelines may continue to evolve.
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What our experts say
Research suggests that diagnostic testing is more accurate a few days after symptoms start, or around a week after exposure to a person who is infected with COVID-19. Testing more than once can confirm negative results, when appropriate, and when tests are available.
During the wait for test results, it is essential for people who suspect they have COVID-19 or have been exposed to COVID-19, to take precautions and self-isolate when possible. It is also important to consider what type of test is being used to check for a COVID-19 infection, as this will likely impact how accurate the test is and how long it will take to get results.
Molecular tests are among the most accurate diagnostic tests currently available for detecting whether someone has an active COVID-19 infection. Molecular tests use methods such as RT-PCR (reverse transcription polymerase chain reaction) to detect genetic material from SARS-CoV-2, the virus that causes COVID-19, in respiratory samples such as nose and throat swabs.
Molecular tests have a higher risk of false negatives in the earliest days after exposure and symptom onset, according to an August 2020 publication in the Annals of Internal Medicine by researchers at John Hopkins University who reviewed 7 published studies on the performance of RT-PCR molecular tests. The researchers found that on average, the false negative rate was lowest around day 8 of an infection or 3 days after symptom onset (symptom onset is typically several days after an infection starts), with the false negative rate rising again as the infection continues. False negative test results in the early stages of infection are concerning, because other research (including studies published in Nature and the American Journal of Pathology) have found that COVID-19 patients can be most infectious to others in the early days of infection, when test results may be more likely to come back as false negatives.
Some testing policies recommend that people get tested twice to confirm a negative result. Repeat testing to confirm negative results can be particularly important for people who may interact with high-risk populations (ex. healthcare workers, caretakers), people who may interact with many others outside of their household (ex. an employee going back to the office, a student returning to in-person classes), and people who may need medical care for COVID-19 (e.g. elderly patients with underlying conditions).
Molecular tests are a relatively accurate type of diagnostic testing, and they have a lower chance of false negatives when conducted a few days after symptoms start, or approximately a week after exposure. A lower chance of false negatives does not mean there is no chance of inaccurate test results, so repeat testing may be recommended to confirm test results in certain situations. With all the ongoing research and development work on COVID-19 tests, pandemic testing guidelines may continue to evolve.
Research suggests that diagnostic testing is more accurate a few days after symptoms start, or around a week after exposure to a person who is infected with COVID-19. Testing more than once can confirm negative results, when appropriate, and when tests are available.
During the wait for test results, it is essential for people who suspect they have COVID-19 or have been exposed to COVID-19, to take precautions and self-isolate when possible. It is also important to consider what type of test is being used to check for a COVID-19 infection, as this will likely impact how accurate the test is and how long it will take to get results.
Molecular tests are among the most accurate diagnostic tests currently available for detecting whether someone has an active COVID-19 infection. Molecular tests use methods such as RT-PCR (reverse transcription polymerase chain reaction) to detect genetic material from SARS-CoV-2, the virus that causes COVID-19, in respiratory samples such as nose and throat swabs.
Molecular tests have a higher risk of false negatives in the earliest days after exposure and symptom onset, according to an August 2020 publication in the Annals of Internal Medicine by researchers at John Hopkins University who reviewed 7 published studies on the performance of RT-PCR molecular tests. The researchers found that on average, the false negative rate was lowest around day 8 of an infection or 3 days after symptom onset (symptom onset is typically several days after an infection starts), with the false negative rate rising again as the infection continues. False negative test results in the early stages of infection are concerning, because other research (including studies published in Nature and the American Journal of Pathology) have found that COVID-19 patients can be most infectious to others in the early days of infection, when test results may be more likely to come back as false negatives.
Some testing policies recommend that people get tested twice to confirm a negative result. Repeat testing to confirm negative results can be particularly important for people who may interact with high-risk populations (ex. healthcare workers, caretakers), people who may interact with many others outside of their household (ex. an employee going back to the office, a student returning to in-person classes), and people who may need medical care for COVID-19 (e.g. elderly patients with underlying conditions).
Molecular tests are a relatively accurate type of diagnostic testing, and they have a lower chance of false negatives when conducted a few days after symptoms start, or approximately a week after exposure. A lower chance of false negatives does not mean there is no chance of inaccurate test results, so repeat testing may be recommended to confirm test results in certain situations. With all the ongoing research and development work on COVID-19 tests, pandemic testing guidelines may continue to evolve.
Context and background
There have been stories circulating in the media about how some people initially tested negative for COVID-19, only to test positive a couple days later. This has raised questions about what types of testing are most accurate, and when (during the course of an infection) is the most accurate time to test someone.
There are currently two main types: 1) diagnostic tests (including molecular tests and antigen tests), also known as viral tests, which indicate whether someone has an active infection; and 2) antibody tests, also known as serology tests, which indicate whether someone was likely to have had a COVID-19 infection in the past. An antibody test is not used to detect whether someone has an active COVID-19 infection, in part because it can take 1–3 weeks after infection for the immune system to produce antibodies.
Molecular tests are considered more accurate, in general, than the newer antigen tests for diagnosing the virus. Antigen tests detect specific proteins on the surface of the virus that causes COVID-19, and are more likely to miss an active infection compared to molecular tests. Antigen tests require repeat testing to confirm negative test results.
While molecular tests are currently considered the most reliable way to detect if someone has an active infection, there can be differences among the different types of molecular tests. For example, newer rapid diagnostic tests can provide results in less than an hour, compared to how most RT-PCR molecular tests provide results in a few hours to a few days. These rapid diagnostic tests are currently not as accurate, however, and researchers are investigating potential issues with the accuracy of rapid diagnostic tests.
There have been stories circulating in the media about how some people initially tested negative for COVID-19, only to test positive a couple days later. This has raised questions about what types of testing are most accurate, and when (during the course of an infection) is the most accurate time to test someone.
There are currently two main types: 1) diagnostic tests (including molecular tests and antigen tests), also known as viral tests, which indicate whether someone has an active infection; and 2) antibody tests, also known as serology tests, which indicate whether someone was likely to have had a COVID-19 infection in the past. An antibody test is not used to detect whether someone has an active COVID-19 infection, in part because it can take 1–3 weeks after infection for the immune system to produce antibodies.
Molecular tests are considered more accurate, in general, than the newer antigen tests for diagnosing the virus. Antigen tests detect specific proteins on the surface of the virus that causes COVID-19, and are more likely to miss an active infection compared to molecular tests. Antigen tests require repeat testing to confirm negative test results.
While molecular tests are currently considered the most reliable way to detect if someone has an active infection, there can be differences among the different types of molecular tests. For example, newer rapid diagnostic tests can provide results in less than an hour, compared to how most RT-PCR molecular tests provide results in a few hours to a few days. These rapid diagnostic tests are currently not as accurate, however, and researchers are investigating potential issues with the accuracy of rapid diagnostic tests.
Resources
- Coronavirus Testing Basics (U.S. FDA)
- Testing for COVID-19 (U.S. CDC)
- The Latest in Coronavirus (COVID-19) Testing Methods and Availability (GoodRx)
- Variation in False-Negative Rate of Reverse Transcriptase Polymerase Chain Reaction–Based SARS-CoV-2 Tests by Time Since Exposure (Annals of Internal Medicine)
- Association of Initial Viral Load in Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Patients with Outcome and Symptoms (American Journal of Pathology)
- Temporal dynamics in viral shedding and transmissibility of COVID-19 (Nature)
- Study Raises Questions About False Negatives From Quick COVID-19 Test (NPR)
- Performance of the rapid Nucleic Acid Amplification by Abbott ID NOW COVID-19 in nasopharyngeal swabs transported in viral media and dry nasal swabs, in a New York City academic institution (bioRxiv)
- Coronavirus Testing Basics (U.S. FDA)
- Testing for COVID-19 (U.S. CDC)
- The Latest in Coronavirus (COVID-19) Testing Methods and Availability (GoodRx)
- Variation in False-Negative Rate of Reverse Transcriptase Polymerase Chain Reaction–Based SARS-CoV-2 Tests by Time Since Exposure (Annals of Internal Medicine)
- Association of Initial Viral Load in Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Patients with Outcome and Symptoms (American Journal of Pathology)
- Temporal dynamics in viral shedding and transmissibility of COVID-19 (Nature)
- Study Raises Questions About False Negatives From Quick COVID-19 Test (NPR)
- Performance of the rapid Nucleic Acid Amplification by Abbott ID NOW COVID-19 in nasopharyngeal swabs transported in viral media and dry nasal swabs, in a New York City academic institution (bioRxiv)
