INTERPRETATION OF COVID-19 RAPID TEST RESULTS

Introduction

DropTech is easy to perform (view guide on how to perform the test) and therefore requires minimal operator training. The test is suitable for deployment in near-patient or point-of-care (POC) settings, including for example, clinical settings. COVID test results using DropTech are easily read at location in designated workplaces or places where close contact with customers is needed.1

The ability of any diagnostic test to achieve satisfactory clinical performance is contingent on it being performed within the appropriate time frame with attention to the principles of good pre-analytical and analytical testing practice. It is therefore important for there to be a clear understanding that a negative result should be interpreted as ‘no virus detected’ rather than ‘not infected with SARS-CoV-2’.2

SARS-CoV-2 swab sample for COVID-19
Rapid antigen test interpretation

Pre-COVID-19 Testing Guidelines

The interpretation of COVID test results should take into consideration the pre-test probability of infection, which is dependent on factors such as local transmission rates, disease prevalence, exposure history and symptoms. Certain information needs to be in place before attempting testing:

  1. Recording the patient’s identifier data (DOB, address, mobile number, and PPS) is mandatory.
  2. A record-keeping system of interpreted results should be in place. 
  3. Establish an appropriate ‘Response Protocol’ to positive test results, including:
    1. Advising immediate self-isolation.
    2. Referring the patient for confirmatory rRT-PCR if appropriate.
    3. Ensure access to the Health Service Executive (HSE) contact tracing team or Health Protection Surveillance Centre (HPSC).

Adhering to safety protocols such as patient flow, specimen collection, and waste disposal as well as governance and record-keeping as outlined in the Guidelines for Safe and Effective Near-Patient Testing 2020

Defining The Test Results

Positive vs. Negative vs. Invalid

Positive test (virus detected): Line appears both in the T and C area.

Negative test (virus not detected): Line appears C area.

Invalid test: Line C is not present even if line T is visible.

Invalid test: No line is present in either area.

Detected vs. Not Detected

A negative DropTech COVID test results indicates the virus is not detected. DropTech has a sensitivity of 92.58%. A negative test therefore does not fully rule out the presence of SARS-CoV-2. A negative test is either:

  • True negative: The patient is not infected.
  • False negative: The patient is infected but displays a negative result. Clinical correlation is advised.

A positive DropTech COVID test results indicates the virus is detected. DropTech has a specificity of 97.58%. A positive result therefore means that infection with SARS-Co-V2 is highly likely.  A positive result is either:

  • True positive: The patient is infected.
  • False positive: The patient is not infected despite a positive result. Confirmatory rRT-PCR will almost certainly clarify the true result.
DropTech rapid antigen COVID-19 test results

Interpretation of Results

A positive COVID-19 test result indicates that the probability of the person being infected with SARS-CoV-2 is high (test specificity >97%). Since viral loads in patients who are asymptomatic and those who have symptoms are similar, a patient who tests positive may be contagious even if they feel well.  All patients with a positive test result should immediately self-isolate and seek further advice from their healthcare provider.

If the test result is negative, this should be interpreted with the clinical scenario in mind. If the pre-test probability of infection is low (high negative predictive value), then a negative Rapid Antigen Detection Test (RADT) may be considered presumptive. For example, someone who is asymptomatic and is not a close contact of a COVID-19 infected person. Close contacts and people with any symptoms should proceed with the confirmatory NAAT, such as real-time reverse transcription polymerase chain reaction (rRT-PCR) even if the RADT is negative. All patients with symptoms should self-isolate and close contacts should restrict their movements according to government guidelines.

Understanding The Possibility of Error

Understanding The Possibility of Error

In some circumstances SARS-CoV-2 may not be detected by rapid antigen detection tests due to pre-analytical or analytical errors such as:

  1. Insufficient viral components at the sampled anatomical site.
    1. Concurrent medications e.g. decongestants. 
    2. Mouth cleansing prior to sampling.
  2. Incorrect sample collection methods.
  3. The timing during infection.
  4. Errors during sample processing.
  5. Test storage errors.
  6. Errors in the reporting of results.

While reading the reaction of the test is simple, interpretation of the meaning of these results is not so straightforward. The interpretation of test results should take potential uncertainties into consideration, and should also consider the probability that an individual is infected based on clinical and epidemiological data to increase the probability of a correct diagnosis.3 Rapid antigen detection tests such as DropTech may be more sensitive within the first 5-7 days of being infected when the viral load is higher. Viral antigen may not be detected in the first 24 hours of infection, so repeat testing may be indicated after two days.4 All patients being administered rapid antigen detection tests must be counseled that they may require repeat testing or confirmatory testing with rRT-PCR which is currently the gold-standard in Ireland. Even when test results are negative, people should continue to wash their hands, wear masks, and practice social distancing. Testing cannot replace the basic control measures that need to be in place to keep COVID-19 under control.

Rapid antigen COVID-19 test report

References

  1. Koczula KM, Gallotta A. Lateral flow essays. Essays Biochem. 2016;60(1):111-20.
  2. World Health Organization. Antigen-detection in the diagnosis of SARS-CoV-2 infection using rapid immunoassays. Interim guidance 11 September 2020. [Available from: https://www.who.int/publications/i/item/antigen-detection-in-the-diagnosis-of-sars-cov-2infection-using-rapid-immunoassays Accessed 14/09/2020 
  3. European Centre for Disease Prevention and Control. An overview of the rapid test situation for COVID-19 diagnosis in the EU/EEA. 1 April 2020. . Stockholm: : ECDC, 20204
  4. Stites EC, Wilen CB. The Interpretation of SARS-CoV-2 Diagnostic Tests. Med (New York, NY). 2020