Unit 5: Choosing an HIV Test

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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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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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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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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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Table 5.1. Comparing EIAs and HIV rapid tests

EIAs 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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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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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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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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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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Table 5.3. Guide for Calculating Sensitivity and Specificity

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  DiseaseTotal

Test result Present Absent

Positive a b a + b

Negative c d c + d

Total a + c b + d a + b + c + d

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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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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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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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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Figures 5.1 and 5.2. Strategies for HIV Testing

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Strategy 1 Strategy 2

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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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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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

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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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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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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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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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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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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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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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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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External Quality Assurance Procedure

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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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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Answers to Warm Up Questions

1. Which of the following is a factor in the decision to select an HIV testing strategy?

a. sensitivity and specificity of test being usedb. objective of the testc. HIV prevalence in the population being testedd. all of the above

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Answers to Warm Up Questions, Cont.

_b_ pre-analytical

_c_ analytical

_a_ post-analytical

a. interpreting results, entering data into tracking system, reviewing quality control

b. training, laboratory safety, selection of test kits

c. specimen processing and storage, analysis of testing performance, reagent preparation

2. Match each phase of the HIV testing process with the components it includes:

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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:

a. quality controlb. quality assurancec. quality performanced. none of the above

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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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Small Group Reports

Select one member from your group to present your answers.

Discuss with the rest of the class.

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Case Study: Instructions

Try this case study individually.

We’ll discuss the answers in class.

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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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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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