Covid-19 testing: which, when and why?

There are now 4 types of Covid-19 tests available at Atlantic Clinic: 

  1. PCR  ultra-fast
  2. Antigen test
  3. Rapid antibody tests for IgM and IgG
  4. ELISA antibody tests for IgM and IgG 

Confused?  Here’s what they mean:

 

1. PCR (polymerase chain reaction): 85€ 

This detects the presence of genetic material from the coronavirus.

How?  Swabs (like a long cotton bud) collect mucous from the back of the throat and nose.

Time for result: from 4 hours

When? The test shows positive early in the disease (within a day or so after symptoms begin).

Why? 

  1. To diagnose the illness in the early stages of the infection, or;
  2. For travel when a negative result is a requirement.

Limitations: 

a. After recovery the PCR may no longer be detectable so it is not so useful to diagnose a previous infection.

b. The PCR may persist for some time even after recovery because of residual genetic material leading to confusion over whether a patient is still contagious.

c. The test may be falsely negative if the sample is not taken correctly, or if the virus is not present in that part of the body (it may be only in the lungs, for example).

Book a test

 

2. Antigen test: 45€

45€ on weekends/out of hours

This detects the presence of the virus in an active infection.

How? A swab collects mucous from the back of the nose.

Time: about 20 minutes.

When? The test shows positive as soon as symptoms start or at least 2 days after contact until 5 days after the symptoms start.

Why?

  1. To diagnose the illness in the early stages of the infection or;
  2. For travel when a negative Antigen test is required

Limitations

The result becomes negative as the infection clears (after about 5 days from the start of symptoms). This may be useful to indicate that a patient is no longer infectious, but it cannot indicate a previous infection (antibody tests are better for this).

Book a test

 

 

 

 3. Rapid antibody tests: 60€ 

This detects the presence of antibodies (the body’s defense against the virus) which are produced in response to the infection.

How? The sample is either a drop of blood from a finger prick or a blood sample taken from the vein. 

Time for the result: At Atlantic Clinic the result is ready the same day.

When? Antibodies take at least 7 days from infection or 4 days from the start of symptoms to show up. The IgM antibodies are seen first as the disease progresses. After recovery, the IgM disappears and IgG persists.

Why? 

  1. It is the quickest and cheapest test.
  2. It is most useful for those who have had no symptoms, or symptoms some time ago, (a minimum of 4 days) and would like to know if:
  •  You have had the disease in the past and have recovered, or;
  • You are currently infected and what stage of infectivity or recovery you are in.

Limitations

Not useful for diagnosis in the early stages of the disease (PCR is better). The test only gives a positive/negative result. Generally the ELISA test is preferred (see below).

Book a test

 

 

 

3. ELISA antibody test (enzyme-linked immunosorbent assay) 95€.

This also detects antibodies but uses more sophisticated techniques and needs skilled technicians. It is more accurate than the rapid test.

How?  A blood sample is taken from the vein. 

Time for result: At Atlantic Clinic the result are ready the same day.

When? As with the rapid test, the antibodies take at least 7 days from infection or 4 days from the start of symptoms to show up. 

Why? It is a more reliable test that the rapid antibody test. It gives a figure showing the amount of IgM and IgG (rather than a simple positive/negative result) which can indicate the degree of immunity. It can used to confirm a rapid antibody test result that is in doubt.

Limitations

  1. It is more expensive than the rapid antibody test and takes longer to get the results.
  2. Not useful for diagnosis in the early stages of the disease (PCR is better).

Book a test

 

 

How accurate are they?

The manufacturers quoted accuracy rates for the methods used at Atlantic Clinic are:

PCR:

Specificity and sensitivity 99.999%

Antigen test:

Specificity 100%, sensitivity: 85%, (this satisfies the requirements for international travel certification)

Rapid antibody test:

– IgM specificity 96% and sensitivity 85%

– IgG specificity 98% and sensitivity 93%

ELISA: 

– IgM specificity 97.5% and Sensitivity 82%

– IgG specificity 97.3% and sensitivity 91.2%

Categories: COVID-19
Post by: Atlantic Clinic on 28 May 2020