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A lot of the information below are derived from the WHO paper on RAT testing - download the paper
There are various ways to test for the presence of Covid viral infection.
1) Nucleic acid amplification tests (NAAT).
- Also called the PCR (real time reverse transcription polymerase chain reaction)
- Expensive, requires expensive diagnostic lab equipment and reagents, multistep procedure
- Slow to get results
- Highly accurate (with certain caveats)
2) Ag-RDT: Antigen rapid detection test / RAT: rapid antigen tests: LFT lateral flow tests - they all mean the same style of test.
- These are accessible, cheaper, rapid tests compared to PCR and can also be reliable and accurate (with certain caveats)
-Most are point of care tests (means you can test where you stand)) and do not require laboratory processing. (some RATs do)
3) Covid antibody testing
Primarily blood test, but this is a historical test looking for past infections, so is of no use in the diagnoses of present Covid infection
Biostatistical intepretation and explanatory terms
1) Sensitivity
- the % of cases which are truly positive that are detected as positive by RAT
2) Specificity
- the % of cases that are truly negative that are detected as negative by RAT
3) Positive predictive value
- the probability that subjects with a positive RAT test truly have the disease
4) Negative predictive value
- the probability that subjects with a negative RAT test do not have the disease.
5) False positive
- an error in which a a RAT test reports a positive result in an individual who is truly negative
6) False negative
- an error in which a RAT test reports a negative result in an individual who is truly positive
See Annex page 17-20 of the attached WHO paper on RAT testing to see examples of the numbers of each of the above category. Pay particular attention to the false positive and false negative.
Accuracy limitation of RAT
WHO recommends RATS with a minimum performance requirements of:
>80% sensitivity
>97% specificity
By comparison the PCR test:
>95% sensitivity
>99.9 specificity
It is important to note that in the asymptomatic population the tests are less accurate:
Less than 50% sensitivity
>97% specificity
RAT perform best in:
People with high viral load, early in the course of the infection
Very reliable where there in ongiong Covid transmission and a Covid prevalence or test positivity rate of >5%
Where there is no or low transmission, the positive predictive value of RAT will be low - it higher false positive rate
In these cases testing should only be done for symptomatic individuals or those asymptomatic individuals at high risk of infection
There is no recommendations for RAT to be used indiscriminately. In fact indiscriminate use lowers positive predictive value of these tests.
Training is recommended for the proper use of RAT
Strict storage and operational temperature requirements are also required.
With summer in Australia, it will not be unusual for a RAT test to sit in the heat inside a parked car.
When should RATS be used
To detect Covid infections in:
- symptomatic individuals early in the course of an illness (less than 5 days after the onset of symptoms)
- the use of RATs later than 7 days after symptoms increases the likelihood of false negatives.
- asymptomatic Individuals at high risk of infection - eg those deemed to be a close contact, health care workers
- in settings where PCR tests are difficult to access
- in negative RATs test in the previous 48hrs but accounting for the epidemiological context, clinical history and presentation
What do the RAT detect
- There are synthetic antibodies on the RAT testing surface which bind to various Covid proteins (the so called antigen) produced by the replicating virus in respiratory secretions/oral saliva.
- covid proteins are produced in different concentrations - nucleocapsid proteins are produced at the highest level while spike proteins are lowest
- Note they do not detect the virus itself.
- mutations might affect test accuracy (spike vs nucleocapsid)
- they do not necessarily detect seroconversion from spike protein vaccinations
Issue of quality testing
- test design : insufficient antibody quantity or affinity to the target antigen (the protein it is testing for)
- cross reactivity with other substances
- heat and humidity (Ill bet that many tests will be sitting in a hot car in summer, or in a handbag in a similar environment)
- adequate sampling
- proper execution of test procedure
RAT Covid testing in asymptomatic populations
The prevalence of Covid in the asymptomatic population is extremely low in the overall tested population.
This meta analysis study suggested 0.25%
However 40% individuals with a confirmed test were asymptomatic
Viral loads are highest very early on in the covid infection - often when the individual is at the cusp of becoming symptomatic.
This means that asymptomatic testing needs to be targeted
RAT Covid testing in asymptomatic individuals who are close contacts of confirmed covid cases
Recommended
If negative may need repeat RAT testing and or PCR testing.
RAT Covid testing in Individuals with symptoms of covid and in the first 5-7 days since onset of symptoms
Yes
If negative, a confirmation RAT test could be performed within 48hrs
A confirmation PCR test should also be done.
RAT Covid testing in the Workplace
Again the general recommendation is testing is recommended in environments where there is a high risk of exposure.
Mass RAT Covid testing
Mass indiscriminate testing of aymptomatic individuals is not recommended
Here are the current TGA approved RAT tests
They are approved in the sense that they conform to the >80% sensitivity and >97% specificity
Again note that the sensitivity in asymptomatic individuals likely to be much lower.
Recommended Storage temperature of RAT tests is generally 1-30C. Especially important with our hot summer above 30% in many places
The intepretation of a positive or negative results depends on various factors including:
- test specificity and sensitivity
- the prevalence of covid in the community based on surveillance
- symptom status of the individual
- clinical context of the individual eg close contact
1) negative tests should be repeated in 24-48hrs depending on the above - but especially if there is high index of suspicion.
If also symptomatic please get a PCR test
2) NSW Health recommends a positive RAT test should be followed up with a confirmatory PCR test but do not get to PCR testing site by public transport/rideshare
- NSW Health can provide Covid Home testing under certain circumstances
- one of the potential problems of a positive RAT is that some will not doa confirmatory PCR and will be effectively hidden from the Health Dept.
With an understanding of the above synopsis, please read the attached pdf in the TGA list in order to see which one is best for you.
A general reminder it is the AFF way to read and in some circumstances, exploit the fine print!!
There are various ways to test for the presence of Covid viral infection.
1) Nucleic acid amplification tests (NAAT).
- Also called the PCR (real time reverse transcription polymerase chain reaction)
- Expensive, requires expensive diagnostic lab equipment and reagents, multistep procedure
- Slow to get results
- Highly accurate (with certain caveats)
2) Ag-RDT: Antigen rapid detection test / RAT: rapid antigen tests: LFT lateral flow tests - they all mean the same style of test.
- These are accessible, cheaper, rapid tests compared to PCR and can also be reliable and accurate (with certain caveats)
-Most are point of care tests (means you can test where you stand)) and do not require laboratory processing. (some RATs do)
3) Covid antibody testing
Primarily blood test, but this is a historical test looking for past infections, so is of no use in the diagnoses of present Covid infection
Biostatistical intepretation and explanatory terms
1) Sensitivity
- the % of cases which are truly positive that are detected as positive by RAT
2) Specificity
- the % of cases that are truly negative that are detected as negative by RAT
3) Positive predictive value
- the probability that subjects with a positive RAT test truly have the disease
4) Negative predictive value
- the probability that subjects with a negative RAT test do not have the disease.
5) False positive
- an error in which a a RAT test reports a positive result in an individual who is truly negative
6) False negative
- an error in which a RAT test reports a negative result in an individual who is truly positive
See Annex page 17-20 of the attached WHO paper on RAT testing to see examples of the numbers of each of the above category. Pay particular attention to the false positive and false negative.
Accuracy limitation of RAT
WHO recommends RATS with a minimum performance requirements of:
>80% sensitivity
>97% specificity
By comparison the PCR test:
>95% sensitivity
>99.9 specificity
It is important to note that in the asymptomatic population the tests are less accurate:
Less than 50% sensitivity
>97% specificity
RAT perform best in:
People with high viral load, early in the course of the infection
Very reliable where there in ongiong Covid transmission and a Covid prevalence or test positivity rate of >5%
Where there is no or low transmission, the positive predictive value of RAT will be low - it higher false positive rate
In these cases testing should only be done for symptomatic individuals or those asymptomatic individuals at high risk of infection
There is no recommendations for RAT to be used indiscriminately. In fact indiscriminate use lowers positive predictive value of these tests.
Training is recommended for the proper use of RAT
Strict storage and operational temperature requirements are also required.
With summer in Australia, it will not be unusual for a RAT test to sit in the heat inside a parked car.
When should RATS be used
To detect Covid infections in:
- symptomatic individuals early in the course of an illness (less than 5 days after the onset of symptoms)
- the use of RATs later than 7 days after symptoms increases the likelihood of false negatives.
- asymptomatic Individuals at high risk of infection - eg those deemed to be a close contact, health care workers
- in settings where PCR tests are difficult to access
- in negative RATs test in the previous 48hrs but accounting for the epidemiological context, clinical history and presentation
What do the RAT detect
- There are synthetic antibodies on the RAT testing surface which bind to various Covid proteins (the so called antigen) produced by the replicating virus in respiratory secretions/oral saliva.
- covid proteins are produced in different concentrations - nucleocapsid proteins are produced at the highest level while spike proteins are lowest
- Note they do not detect the virus itself.
- mutations might affect test accuracy (spike vs nucleocapsid)
- they do not necessarily detect seroconversion from spike protein vaccinations
Issue of quality testing
- test design : insufficient antibody quantity or affinity to the target antigen (the protein it is testing for)
- cross reactivity with other substances
- heat and humidity (Ill bet that many tests will be sitting in a hot car in summer, or in a handbag in a similar environment)
- adequate sampling
- proper execution of test procedure
RAT Covid testing in asymptomatic populations
The prevalence of Covid in the asymptomatic population is extremely low in the overall tested population.
This meta analysis study suggested 0.25%
However 40% individuals with a confirmed test were asymptomatic
Viral loads are highest very early on in the covid infection - often when the individual is at the cusp of becoming symptomatic.
This means that asymptomatic testing needs to be targeted
RAT Covid testing in asymptomatic individuals who are close contacts of confirmed covid cases
Recommended
If negative may need repeat RAT testing and or PCR testing.
RAT Covid testing in Individuals with symptoms of covid and in the first 5-7 days since onset of symptoms
Yes
If negative, a confirmation RAT test could be performed within 48hrs
A confirmation PCR test should also be done.
RAT Covid testing in the Workplace
Again the general recommendation is testing is recommended in environments where there is a high risk of exposure.
Mass RAT Covid testing
Mass indiscriminate testing of aymptomatic individuals is not recommended
Here are the current TGA approved RAT tests
They are approved in the sense that they conform to the >80% sensitivity and >97% specificity
Again note that the sensitivity in asymptomatic individuals likely to be much lower.
Recommended Storage temperature of RAT tests is generally 1-30C. Especially important with our hot summer above 30% in many places
The intepretation of a positive or negative results depends on various factors including:
- test specificity and sensitivity
- the prevalence of covid in the community based on surveillance
- symptom status of the individual
- clinical context of the individual eg close contact
1) negative tests should be repeated in 24-48hrs depending on the above - but especially if there is high index of suspicion.
If also symptomatic please get a PCR test
2) NSW Health recommends a positive RAT test should be followed up with a confirmatory PCR test but do not get to PCR testing site by public transport/rideshare
- NSW Health can provide Covid Home testing under certain circumstances
- one of the potential problems of a positive RAT is that some will not doa confirmatory PCR and will be effectively hidden from the Health Dept.
With an understanding of the above synopsis, please read the attached pdf in the TGA list in order to see which one is best for you.
A general reminder it is the AFF way to read and in some circumstances, exploit the fine print!!
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