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Infectious Diseases 2021

Issues around testing for early viral infections

Dr Julie Fox

Reader of HIV Medicine, Kings College London

Prof. Sarah Fidler

Professor of HIV Medicine, Imperial College London

Diagnosing infection early is important to initiate containment through contact tracing and behavioural modification, as well as allow timely introduction of treatment interventions.

Similar themes are relevant across infections; we are focussing on the examples of HIV and SARS-CoV-2 to discuss this.

The early diagnosis of a viral infection can be affected by the following factors:

1. Time to detect virus after infection varies:

Different viruses replicate at varying rates after infection. This affects how quickly a test can detect viral proteins or genes. For SARS-CoV-2 this is approximately five days, whereas for HIV it is approximately two weeks.

2. It is common to have detectable virus and be infectious before developing symptoms.

It is common to have detectable virus and be infectious before
developing symptoms; for SARSCoV-2 this is approximately five days and, for HIV, can last years. This means that only testing symptomatic people can miss a large pool of infection.

3. Diagnostic tests can be negative very early in infection:

The time from exposure to when a test can reliably detect infection is critical to timely diagnosis. Routine HIV testing detects antibody which usually develops six to eight weeks following exposure. Detecting early infection therefore requires detection of viruses prior to the development of an antibody, either by testing directly for virus or by testing for the p24 virus protein. The latter can be done using finger-prick blood samples using a lateral flow rapid test. For SARS-CoV-2 diagnosis, antibodies develop between 6–15 days after the onset of symptoms  (time from exposure is not known), and at this point an individual is no longer infectious. To detect active infection, nasal or throat swabs which detect either virus or antigen are used.

4. Rapid point of care tests:

These types of tests do not require laboratory equipment and provide a quick result. For HIV tests, result time ranges from 1 to 20 minutes for SARS-CoV-2, lateral flow antigen results range from 15 to 30 minutes..

5. No test is 100% accurate:

For a test to be introduced, it must be highly sensitive to detect low levels of virus, specific to not make false positive tests and ideally be cheap to allow mass roll out.

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