Equivocal Results are related to EPIC Order for SARS-Cov-2 RNA Diagnostic PCR Test LAB8572.
What does an equivocal result mean?
“Equivocal” means results cannot be interpreted as positive or negative because they are below the limit of detection (see below). Reasons may be:
- Very early or late in infection
- Suboptimal sample collection
- Lower respiratory tract disease only
- Resolved past infection (< 270 days from symptom onset)
- Analytical false positive
What should I do if my patient has an equivocal result? Do they need isolation?
Assess the patient for COVID-19 symptoms.
Asymptomatic patients: If it has been more than 24 hours since the equivocal test was collected and the patient remains asymptomatic, the result is not reflective of an active COVID-19 infection. Contact the infection preventionist on call and the COVID-19 isolation flag will be removed on the basis of this information.
If a patient has symptoms:
Additional clinical assessment and history-taking would be required to determine if the result in is false positive vs reflective of RNA detection from a long-resolved infection.
The type and duration of symptoms, COVID-19 exposures, vaccination status, and number of preceding negative or positive COVID-19 PCR tests will help in the evaluation of the patient’s equivocal test.
For patients with acute symptoms after or at the time of an equivocal test result, repeat testing may be recommended in the following 24-48 hours which would be expected to be markedly positive if the original test was a true acute COVID-19 infection with pre-symptomatic detection vs symptoms from an unrelated infection/condition.
Contact infection prevention on call who will ask for the above information to work out a plan to remove or continue isolation status for your patient.
A cycle threshold value (Ct or CN) is the cycle of amplification that the fluorescence in a PCR reaction becomes positive. MUSC’s COVID-19 PCR tests are highly sensitive with a limit of detection around cycle 38, a limit established by repeatedly testing reference standards. The MUSC molecular pathology laboratory conducted an internal study looking at the reproducibility of patient samples with a crossing threshold of greater than 38 and found that 80% of the positive samples with a CN greater than 38 repeated as negative. Given the unreliability samples with CN values >38, the laboratory and Infection Prevention Control have agreed that any results with greater than 38 cycle thresholds should be called equivocal rather than positive. By comparison, most acute COVID-19 infections have a CN of 12-20.
Beginning 10/5/2021, COVID-19 tests with CN>38 on the COVID-19 PCR test (LAB8572) will be posted to end users with an equivocal result (in addition to the existing positive, negative, and invalid results). When a patient’s test is equivocal, it results from: (1) pre-symptomatic detection when viral loads are low for 24 hours before symptom onset, (2) resolved past infection (up to 270 days from symptom onset), or (3) false positive. Only clinical evaluation of the patient can tell which of these is likeliest.