Thursday, 4 April 2013

RAPID TESTING FOR HIV DIAGNOSIS



By Geoffrey Oguma, SMP Lab Technologist


INTRODUCTION
HIV rapid serology testing remains the cheapest and most common means of establishing HIV sero – status for individuals two years or more in age.  Rapid HIV tests are immunological tests based on the principle of an HIV – specific antigen – antibody reaction.  HIV is composed of several proteins and infection will trigger formation of specific antibodies; HIV antibodies will be found in all HIV – infected patients when chronic infection is established.

There are several rapid test kits/devices available.  These include; Unigold recombigen, STAT PACK, MULTISPOT and Clear view Complete.  Most of the rapid tests are able to detect both HIV 1 and 2.  Rapid HIV testing is widely used for point of care under Provider Initiated Counseling and Testing (PICT), Routine Counseling and Testing (RCT) and Voluntary Counseling and Testing (VCT) in static clinics and during outreaches.  The rapid test kits can be used on different specimen such as whole blood, serum saliva and urine.  The fact that rapid tests are generally highly sensitive, provide quick results and do not require highly technical expertise testing has made them the tests of choice in high HIV prevalence and resource limited settings like Uganda.

HIV TESTING ALGORITHMS
Although rapid tests generally have high sensitivity (capacity to correctly identify individuals infected with HIV), some may be more specific (specificity is the capacity to correctly identify individuals not infected with HIV) than others.  With these considerations and knowledge of the prevalence levels in the population and other practical issues, the World Health Organization recommends use of several rapid tests simultaneously or serial testing.

The Ministry of Health in Uganda recommends the use of at least two rapid tests as shown below:
In Uganda, the rapid tests of choice in the algorithm are as follows:
First Testing (Screening)               Determine
Second Test (Confirmatory)          STAT PACK
Third Test (The breaker)               Unigold  

It is important to understand the HIV rapid testing basic principles, interpretation of the test and to undertake necessary quality assurance measures in order to come up with a meaningful result that can be used to guide care and treatment intervention.  A poorly carried out or interpreted HIV test not only causes confusion and distress to the client tested but can also result in poor medical interventions.

CASE SCENARIO
A pregnant woman took an HIV test in May 2011 at a health facility during antenatal visit and was reported HIV positive; she was enrolled into PMTCT (Option A) services in higher health facility.  She delivered in January 2012.  DNA PCR and HIV serology for the baby were negative.  The mother was retested and she was found to be HIV sero – negative.

QUESTION
Is this woman HIV positive or negative?  What could have gone wrong with the first test?  What should the laboratory personnel always do to avoid such confusion?  What should the clinician do for this client?

Good understanding of the HIV testing protocol is critical to minimize errors in HIV diagnosis.  In the above case scenario, a positive result would mean that the mother received ART unnecessarily, while a false negative would mean that she would miss the PMTCT intervention, increasing the risk of HIV transmission to the baby.

FALSE HIV RESULTS
There are several factors that lead to false positive HIV test.  Technical errors are the most common, which can be as a result of improper specimen collection, labeling, storage and preparation which may contaminate specimens, use of expired or defective testing devices, incorrect sample type or volume dispensed to the test devices, wrong reagents/buffers, wrong readings and wrong documentation of results.  However, there are some documented causes of false positive HIV test which include Epstein – Barr virus infection, pregnancy, receipt of HIV vaccine, high serum bilirubin levels and certain autoimmune diseases.

Similarly, false negative results can be caused by technical errors above.  They can also be due to early HIV infection in the acute phase (window period) before antibodies are produced; this window period typically takes between 3 – 12 weeks.

On retrospective review of records of this mother’s medical records (under Case Senario above), it was found that only one HIV kit “determine” had been used as other kits had been out of stock and no follow up testing was advised or done.

Therefore, this was inadequate testing in the 1st place with insufficient
 reporting of results.  No checks were in place at the second health unit; no repeat testing or review of referral notes were observed.  This mother received unnecessary ART with its potential side effects and possibly negative psychosocial implications as well.
PRACTICAL LABORATORY CONSIDERATIONS WHEN DOING HIV RAPID TESTS
Sample Collection:     
1)   Obtain the right specimens for the test kits used and observe safety and infection prevention of the rules of the testing area.
2)   Have SOPs in place for sample collection, packaging and transportation in relation to rapid testing, quality control (QC) and QA;
TEST KITS
1)   Use the right kits for the recommended and adopted testing a logarithm.
2)   Keep the testing devices at the right temperature (2 – 8 degrees centigrade for Elisa reagents or room temperatures for the rapid test kits).
3)   Check the expiry dates and integrity of the kits to ensure they are not expired or damaged before use.
For indeterminate results: double check and rule out possible contamination and report results accordingly.  Encourage repeat testing at 3 and 6 months respectively.  If resources permit, do a confirmatory test that uses a different testing modality.  Finally have a QA/QC system in place to ensure that all is well in the testing process.
CONCLUSION
As rapid HIV testing has become widely available as the major HIV diagnostic tool, there is need to ensure that it is properly performed to give the right result to avoid the untoward implications of a false positive or false negative HIV result.
Source: ATIC Newsletter. Volume 9; Issue 2 of December 2012
A Quarterly Newsletter of the AIDS Treatment Information Centre,
Infectious Diseases Institute,
Makerere University
ATIC TOLL FREE NUMBER: 0800200055 OR
+256717326500

1 comment:

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