This week we have started to do elective procedures and many people have asked about the screening and testing process for COVID-19. In addition to asking people whether they have the following symptoms: fever or chills, cough, shortness of breath or difficulty breathing, fatigue, muscle or body aches, headache, new loss of taste or smell, sore throat, congestion or runny nose, nausea or vomiting, or diarrhea we have been testing for active viral shedding with a swab test. There are two categories of tests currently available for COVID -19. The first tests for the active presence of the virus from a nasal or pharyngeal swab to see if there is active shedding of virus. A positive test means there is active infection and self-quarantine is recommended if only mild symptoms are present. In our specific use of the test it also means any procedures will be postponed until a future negative test after recovery. The second test is a blood or serum antibody test to access the immune response to see if there is an active or if there had been a recent viral infection. If an antibody is found in the blood, it usually means that there was exposure to the virus and antibodies where created. There are acute or immediate antibodies such as IgM and slower developing and usually longer lasting protective antibodies like IgG, which form in the convalescent or recovery phase of an infection. Unfortunately, the current blood tests do not distinguish between IgM and IgG so having a total of IgG antibody (includes IgG and IgM) as a result will not distinguish between an active acute infection and recent or past infection. Furthermore interpretation of a positive or negative test needs to be taken the context of the prevalence or number of people that have been infected by the virus.
Recently the CDC came out with some guidelines which have been publicized in a CNN article as well. Unlike the nasal swab test which is a little more straight forward (except for the recent Abbot rapid test which we won’t cover here), the interpretation of the serum antibody test requires a few more considerations. Let me explain here.
At present, we believe the prevalence is 4-5% percent of the population in California. This number may need to updated as more testing his done. The blood test has a sensitivity rate of 90% and a specificity rate of 95%. The higher the sensitivity, the fewer false negatives a test will give. The higher the specificity, the fewer false positives. There is the concept of positive predictive value and negative predictive value that needs consideration as well. The positive predictive value reflects how accurate and useful is a positive result; whether it is a true positive rather than a false positive result. The negative predicative value reflects how accurate and useful is a negative result; whether you can say the result is negative and there truly has been no exposure or a true negative. Recent CDC guidelines have given few examples to illustrate these concepts. "For example, in a population where the prevalence is 5%, a test with 90% sensitivity and 95% specificity will yield a positive predictive value of 49%. In other words, less than half of those testing positive will truly have antibodies," as quoted in the CDC report. The corollary however, is that with these same numbers the negative predictive value is 99%. The math is not complicated but takes a little time to show so I’ll skip that for now. So this means if you get a negative test for the antibody test at the present estimated prevalence rate of 5% in California we are 99% certain you have not been exposed or had virus.
"Alternatively, the same test in a population with an antibody prevalence exceeding 52% will yield a positive predictive greater than 95%, meaning that less than one in 20 people testing positive will have a false positive test result." Interesting the negative predictive value drops a little to 90%. So prevalence or how rampant the disease or infection is present in a given population is a big factor in how accurate your serologic or blood test will be when it comes to this antibody test. When you have a disease or infection with low prevalence like 5% here in California, one should try for a test with higher specificity and perhaps choose a subset of the population with a higher potential for exposure.
In the present situation, if a blood or serum tests comes back positive, it may represent a recent past infection, an active infection (especially if symptoms), or a false positive (about half the time with a 5% prevalence rate). A confirmatory nasal swab test would be helpful to determine if active shedding virus were present which would indicate a current active infection and would require self-quarantine with mild symptoms. If the nasal swab test were negative and there are no symptoms then no further testing at present is recommended. If there were symptoms then the usual medical work up would proceed as normal per the symptoms.
The facilities were I am doing procedures have been taking temperatures and asking screening questions of everyone involved including physicians, staff, and patients. I have taken the additional step of testing for COVID-19 as I’ve just outlined. This is per guidelines consistent with our local university centers like the University of San Francisco. I have recently tested negative with the pharyngeal swab test. We are taking every precaution with personal protective equipment for everyone involved in patient care and are following current guidelines.
As an announcement, I will be doing a webinar on immune health and the gut Friday, June 19, 2020 at 4PM. Please let me know if you are interested or have any questions might me to address at email@example.com. I’ll be emailing more information in a future email.
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We continue offer online functional medicine assessment, HIPPA compliant video or phone telehealth visits, and in home functional lab kits that you can mail back directly to the laboratories. We can work together to address your health challenges in the comfort and safety of your home during COVID-19 shelter in place and beyond.