Unit 5:
Choosing an HIV Test
#3-5-1
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.
#3-5-2
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
#3-5-3
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
#3-5-4
Selecting an HIV Antibody Test

Currently a wide range of different HIV
antibody tests are available, including:

Conventional enzyme immunoassay (EIA)

Rapid tests
#3-5-5
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 Less skill required
required
Equipment
requirements
Requires complex
equipment,
maintenance
None to minimal
equipment
Storage
Test kits require
refrigeration
Most test kits stored
at room temperature
#3-5-6
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
#3-5-7
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
#3-5-8
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:
#3-5-9
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
#3-5-10
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
Total
#3-5-11
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
#3-5-12
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
#3-5-13
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
#3-5-14
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
#3-5-15
Figures 5.1 and 5.2. Strategies for
HIV Testing
Strategy 1
Strategy 2
#3-5-16
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
#3-5-17
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.
#3-5-18
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
#3-5-19
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
#3-5-20
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
#3-5-21
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.
#3-5-22
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
#3-5-23
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
#3-5-24
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.
#3-5-25
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.
#3-5-26
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.
#3-5-27
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.
#3-5-28
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.
#3-5-29
Answers to Warm Up Questions
1. Which of the following is a factor in the
decision to select an HIV testing strategy?
a.
b.
c.
d.
sensitivity and specificity of test being used
objective of the test
HIV prevalence in the population being tested
all of the above
#3-5-30
Answers to Warm Up Questions,
Cont.
2. Match each phase of the HIV testing process with the
components it includes:

_b_ pre-analytical
a. interpreting results, entering
data into tracking system,
reviewing quality control

_c_ analytical
b. training, laboratory safety,
selection of test kits
c.

_a_ post-analytical
specimen processing and
storage, analysis of testing
performance, reagent
preparation
#3-5-31
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.
b.
c.
d.
quality control
quality assurance
quality performance
none of the above
#3-5-32
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.
#3-5-33
Small Group Reports

Select one member from your group to
present your answers.

Discuss with the rest of the class.
#3-5-34
Case Study: Instructions

Try this case study individually.

We’ll discuss the answers in class.
#3-5-35
Case Study Review

Follow along as we go over the case study in
class.

Discuss your answers with the rest of the
class.
#3-5-36
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?
#3-5-37