The Abbott and Roche tests are supposedly very accurate and both require a blood draw at a lab. Is this failure rate for the finger prick tests and are these the tests that cities are using to determine the # with antibodies in their populations?
Yea I remember that that's why I was kind of surprised. The article doesn't really get into those tests though sounds just like an overall assessment. It seems like the veracity is related more to the prevalence of the virus in a community. The higher the suspected prevalence the more likely the results are to be true, the lower the prevalence the less likely and a second follow up test should be done to verify. Notice the part I bolded in the snippet from the article.
@RU848789 I think pointed this out as well.
From the article:
Antibody tests used to determine if people have been infected in the past with Covid-19 might be wrong up to half the time, the US Centers for Disease Control and Prevention said in
new guidance posted on its website.
Antibody tests, often called serologic tests, look for evidence of an immune response to infection. "Antibodies in some persons can be detected within the first week of illness onset," the CDC says.
They are
not accurate enough to use to make important policy decisions, the CDC said.
"Serologic test results should not be used to make decisions about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities," the CDC says.
Serologic test results should not be used to make decisions about returning persons to the workplace."
Health officials or health care providers who are using antibody tests need to use the most accurate test they can find and might need to test people twice, the CDC said in the new guidance.
"In most of the country, including areas that have been heavily impacted, the prevalence of SARS-CoV-2 antibody is expected to be low, ranging from less than 5% to 25%, so that testing at this point might result in relatively more false positive results and fewer false-negative results," the CDC said.
The higher the sensitivity, the fewer false negatives a test will give. The higher the specificity, the fewer false positives. Across populations, tests give more accurate results if the disease being tested for is common in the population. If an infection has only affected a small percentage of people being tested, even a very small margin of error in a test will be magnified.
If just 5% of the population being tested has the virus, a test with more than 90% accuracy can still miss half the cases.
It's a point that's been made frequently in recent weeks by public health experts, but the CDC spells out the problem in the new advice on antibody testing.
Snippet from CDC guidelines.
Test performance
The utility of tests depends on the sensitivity and specificity of the assays; these performance characteristics are determined by using a defined set of negative and positive samples. In addition, the predictive values of a test should be considered because these values affect the overall outcome of testing. Positive predictive value is the probability that individuals with positive test results are truly antibody positive. Negative predictive value is the probability that individuals with negative test results are truly antibody negative. Positive and negative predictive values are determined by the percentage of truly antibody positive individuals in the tested population (prevalence, pre-test probability) and the sensitivity and specificity of the test. For example:
- In a high-prevalence setting, the positive predictive value increases — meaning it is more likely that persons who test positive are truly antibody positive – than if the test is performed in a population with low-prevalence. When a test is used in a population where prevalence is low, the positive predictive value drops because there are more false-positive results, since the pre-test probability is low.
- Likewise, negative predictive value is also affected by prevalence. In a high-prevalence setting, the negative predictive value declines whereas in a low-prevalence setting, it increases.
In most of the country, including areas that have been heavily impacted, the prevalence of SARS-CoV-2 antibody is expected to be low, ranging from <5% to 25%, so that testing at this point might result in relatively more false positive results and fewer false-negative results.
In some settings, such as COVID-19 outbreaks in food processing plants and congregate living facilities, the prevalence of infection in the population may be significantly higher. In such settings, serologic testing at appropriate intervals following outbreaks might result in relatively fewer false positive results and more false-negative results.
https://www.cdc.gov/coronavirus/2019-ncov/lab/resources/antibody-tests-guidelines.html