1. Unit 5: Choosing an HIV Test
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2. Warm Up Questions: Instructions
Take five minutes now to try the Unit 5 warm up questions in your manual.
Please do not compare answers with other participants.
Your answers will not be collected or graded.
We will review your answers at the end of the unit.
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3. What You Will Learn By the end of this unit you should be able to:
describe the advantages and disadvantages of different HIV testing options
describe how to choose a strategy for HIV testing
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4. What You Will Learn, Cont.
By the end of this unit you should be able to:
understand the difference between sensitivity and specificity of a laboratory test
identify the phases of the testing process, and what quality control and quality assurance programmes should be implemented in each phase
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5. Selecting an HIV Antibody Test
Currently a wide range of different HIV antibody tests are available, including:
Conventional enzyme immunoassay (EIA)
Rapid tests
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6. Table 5.1. Comparing EIAs and HIV rapid tests
Time to result
>60 minutes
10-30 minutes
Testing volume
Suitable fro large volume and batch testing
Suitable for small and large volumes
Staff requirements
Skilled technical staff required
Less skill required
Equipment requirements
Requires complex equipment, maintenance
None to minimal equipment
Storage
Test kits require refrigeration
Most test kits stored at room temperature
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7. Selecting an HIV Testing Algorithm
UNAIDS and WHO recommend three criteria for choosing and HIV testing algorythm or strategy:
objective of the test (surveillance, blood screening, etc)
sensitivity and specificity of the test(s) being used
HIV prevalence in the population being tested
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8. Table 5.2. Selection of HIV Testing Algorithms for Surveillance
Patient has the disease
Patient does not have the disease
Positive test result
True positive
False positive
Negative test result
False negative
True negative
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9. Sensitivity and Specificity
Sensitivity refers to the ability of a test to detect all persons with a disease:
Specificity refers to a test’s ability to detect all persons who do not have a disease:
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10. Positive and Negative Predictive Value
Positive predictive value: proportion of positive tests that identify people who truly have a disease
The more frequent a disease is in a population, the higher the positive predictive value of a test is.
Negative predictive value: proportion of negative tests that identify people who truly do not have a disease
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11. Table 5.3. Guide for Calculating Sensitivity and Specificity
Disease
Total
Test result
Present
Absent
Positive a b
a + b
Negative c d
c + d
a + c
b + d
a + b + c + d
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12. Testing Strategy 1 Requires one test
For use in diagnostic testing in populations with an HIV prevalence >30% among persons with clinical signs or symptoms of HIV infection
For use in blood screening, for all populations regardless of the estimated prevalence in population being tested
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13. Testing Strategy 1, Cont.
For use in surveillance testing in populations with an HIV prevalence >10% (for example, unlinked anonymous testing for surveillance among pregnant women at ANCs)
No results are provided
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14. Testing Strategy 2 Requires up to two tests
For use in diagnostic testing in populations with an HIV prevalence ≤ 30% among persons with clinical signs or symptoms of HIV infection or >10% among persons with no such signs or symptoms
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15. Testing Strategy 2, Cont.
For use in surveillance testing in populations with an HIV prevalence ≤ 10% (for example, unlinked anonymous testing for surveillance among pregnant women at ANCs or patients at STI clinics)
No results are provided
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16. Figures 5.1 and 5.2. Strategies for HIV Testing
Strategy Strategy 2
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17. Selection of HIV Testing Technologies
Decide which testing format to use:
an enzyme linked immunoassay (EIA),
rapid EIA testing, or
a combination of the two
Conditions to consider include:
laboratory infrastructure
availability of skilled laboratory personnel
existence of quality assurance and control measures
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18. Laboratory Testing
The laboratory tests now in use have sensitivities and specificities of 99% or more.
In sub-Saharan Africa, HIV tests performed must be able to detect the presence of antibodies to both HIV-1 and HIV-2.
An additional test is required to distinguish between HIV-1 and HIV-2.
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19. Quality Control
Quality control: process for running the specimens (such as using positive and negative controls), done in the lab facility itself
To verify test device is accurate, external positive and negative controls must be tested.
The test kit manufacturer or a reference laboratory can provide these controls.
Positive controls: specimens known to be positive
Negative controls: are specimens known to be negative
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20. Quality Assurance
Quality assurance: process for comparing results obtained for a specific test with other tests done on the same specimen, done by the lab itself or by outside reference laboratory
Laboratories should monitor and assess quality during three phases of the testing process:
pre-analytical: activities that occur before a specimen is tested
analytical: the actual testing of the specimen
post-analytical: activities done after a specimen has been tested
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21. Table 5.5. Quality Assurance: Pre-Analytical Phase
Training
Laboratory safety
Number of trained personnel available and capable of performing HIV testing
Specimen collection, labelling and transport conditions
Deciding on handling of specimens before testing
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22. Table 5.5. Quality Assurance: Pre-Analytical Phase, Cont.
Deciding on the sources and types of specimens to be tested
Deciding on the number of specimens tested
Selection of test kits
Expiration dates of test kits. Kits need to be used before expiration dates. Older kits should be used before newer kits.
HIV test kit reagents. Reagents must be stored at the appropriate temperature as specified by the manufacturer.
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23. Table 5.5. Quality Assurance: Analytical Phase
Specimen processing and storage
Written procedure manual
Reagent preparation
Testing performance
Performance and maintenance of equipment
Correct use of reagents
Inclusion of internal quality controls in test kits
Quality control monitoring procedure
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24. Table 5.5. Quality Assurance: Post-Analytical Phase
Interpreting results
Transcribing results, such as recording results on the correct identifier code
Entering data into the tracking system (computer or hard copy)
Maintaining records
Reviewing quality control
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25. Internal Quality Assurance
Internal quality assurance is meant to allow laboratory technicians to check their performance for themselves:
An aliquot of every twentieth negative and every fifth positive specimen is put aside.
Once there are sufficient stored aliquots, stored specimens are tested a second time.
Lab technicians compare initial results and results of re-testing to monitor reliability of techniques.
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26. External Quality Assurance
Countries should require that all laboratories at all levels participate in an external quality assurance programme.
This may be instituted either by a national or international reference laboratory.
Proficiency testing should be done 1-2 times a year.
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27. External Quality Assurance, Cont.
In areas with limited infrastructure, labs can prepare a dried blood spot on filter paper to be tested at the national reference laboratory.
External quality assurance for national reference laboratory should be provided by an independent laboratory or one of WHO’s regional quality assurance programmes.
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28. External Quality Assurance Procedure
National reference laboratory sends to all participating laboratories a proficiency panel of 6 specimens to identify as HIV- or HIV+.
Panels are tested at local laboratories in the same way as they routinely test their specimens for HIV.
Local laboratories report their findings to the reference laboratory, which collates results and provides feedback.
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29. Warm Up Review
Take a few minutes now to look back at your answers to the warm up questions at the beginning of the unit.
Make any changes you want to.
We will discuss the questions and answers in a few minutes.
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30. Answers to Warm Up Questions
Which of the following is a factor in the decision to select an HIV testing strategy?
sensitivity and specificity of test being used
objective of the test
HIV prevalence in the population being tested
all of the above
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31. Answers to Warm Up Questions, Cont.
2. Match each phase of the HIV testing process with the components it includes:
_b_ pre-analytical
_c_ analytical
_a_ post-analytical
interpreting results, entering data into tracking system, reviewing quality control
training, laboratory safety, selection of test kits
specimen processing and storage, analysis of testing performance, reagent preparation
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32. Answers to Warm Up Questions, Cont.
3. The process by which reference specimens are tested externally to assure accuracy of a technician’s or laboratory’s performance is known as:
quality control
quality assurance
quality performance
none of the above
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33. Small Group Discussion: Instructions
Get into small groups to discuss these questions.
Choose a speaker for your group who will report back to the class.
Take 15 minutes for this exercise.
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34. Small Group Reports
Select one member from your group to present your answers.
Discuss with the rest of the class.
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35. Case Study: Instructions
Try this case study individually.
We’ll discuss the answers in class.
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36. Case Study Review Follow along as we go over the case study in class.
Discuss your answers with the rest of the class.
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37. Questions, Process Check
Do you have any questions on the information we just covered?
Are you happy with how we worked on Unit 5?
Do you want to try something different that will help the group?
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