The Spain Ministry of Health’s Covid-19 early detection, surveillance and control strategy, includes tests for lgM and lgG antibodies specific to SARS-CoV-2 for the diagnosis of this disease.
The lgM antibody can be detected 7 days after infection or on the third day after the onset of symptoms with a rapid test, which will be useful for screening active infections with a subsequent confirmatory diagnosis by PCR. While IgG antibodies appear later and confer longer-term immunity. IgG positivity may be an indicator of an acute infection in resolution or a previous infection already resolved.
In general, the performance of serological tests is more sensitive in patients who present with symptoms for more than 1 week, but it can give us valuable information even on asymptomatic people, which is why its use is becoming widespread and contemplated in different areas in addition to studies of seroprevalence:
Hospitals. For rapid response in the Emergency Department, for patients with more than 7 days with symptoms, for control and follow-up of health professionals, for screening for other causes, for confirmation of negative PCR with symptoms, to confirm discharges after positive PCR without further symptoms.
The measurement of antibodies in patients with symptoms for several days avoids the risk of exposure of healthcare professionals by not having to take as many samples with swabs, while allowing the PCR technique to be reserved for early detection.
Primary care: Confirm cases in patients with symptoms for more than 7 days, contact tracing, confirm isolation after positive PCR without further symptoms, confirm past infection. Cases of early seroconversion have been shown to exist, so their use in conjunction with PCR can increase the performance of the contact tracing technique.
Senior residences, cohabiting communities and prisons. Periodic controls of all inmates for case screening.
Labor sphere . Control of recent or past infection and periodic monitoring of workers with a high risk of infection such as: police, civil guard, firefighters, ambulance drivers, funeral homes, civil protection, sanitary waste management companies, pharmacists, dentists, cleaning companies and disinfection, etc.
Vulnerable people: Regular checks to detect positives early in the disabled, pregnant, chronically ill.