HIV test

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An HIV test is a procedure that can be used to determine if a person or a donated blood is infected with human immunodeficiency virus (HIV). With the help of HIV tests, the infection can be detected in serum , plasma , saliva or urine . They detect antigens or RNA from HIV or antibodies made by the body against HIV.

In HIV tests, a distinction is made between screening tests and confirmatory tests. The aim of a search test (e.g. ELISA search test) is to identify as many infected people as possible. It is accepted that some uninfected people will also be wrongly tested positive (anti-HIV reactive). Accordingly, search tests need a high sensitivity , but have a relatively low specificity .

If a person tests positive in the addiction test, a confirmation test (in Germany and the USA: Western blot confirmation test) is required in many countries in order to rule out a false-positive diagnosis. A confirmatory test therefore needs a higher specificity than a search test.

HIV tests are usually done in a laboratory. For some years now, point-of-care tests ("rapid tests ") have also been in existence that show a result after just half an hour of adding blood. Rapid tests are used in developing countries that lack well-equipped laboratories and in situations where a quick result is important, e.g. B. for the decision on post- exposure prophylaxis after needle stick injuries .

The AIDS service centers and health authorities in many major German cities offer rapid tests with advice and results after 30 minutes . Self-tests for HIV have been allowed to be freely sold in Germany since October 2018, for example in pharmacies, drug stores and on the Internet. Such self-tests have been available in pharmacies in Austria since July 2018.

frequency

Everyone is advised to have an HIV test annually. If there are more than 10 different sexual partners in twelve months, it is recommended that a test be done every 2 to 3 months. Tests for other sexually transmitted diseases should also be done regularly according to your own risk.

40% of the 16–65 year old population in Germany said they had ever taken an HIV test, 11% in the last 12 months. As of 1999, the proportion of those who had ever taken a test was over 20%, since 2002 it has been / over 30%. The proportion of those tested in the last 12 months has been recorded since 1994 and is between 8 and 11 percent each year.

Approximately 2 million blood and plasma donors are routinely tested for HIV with each donation.

About 90% of all pregnant women in Germany are tested for HIV, which amounts to about 550,000 HIV tests per year (as of 2009, rate and number have been increasing continuously since 2001).

In a 2010 study of 50,000 men who have sex with men (MSM) in Germany, 70% stated that they had ever taken an HIV test, of which half had taken one in the last 12 months. Among MSM under 25 years of age, 47% said they had ever taken a test, 67% of them within the last 12 months.

About half of those diagnosed with HIV could have been helped better if the infection had been detected earlier.

In Germany around 2 million HIV tests are carried out every year, mostly in connection with donating blood or blood plasma. 130,000 tests / year take place at health authorities or AIDS organizations. In 1988 it should have been three million tests.

Search test using immunoassay

How the immunassay works

Automated immunassays (e.g. CMIA, ECLIA, FPIA) or, in isolated cases, the classic enzyme linked immunosorbent assay (ELISA) are the most common screening methods for detecting HIV in the human body. Patients who ask for clarification about an HIV infection in Germany will be examined using an immunoassay, unless they insist on another examination method. For the most part, HIV tests are completed in routine laboratories on the day the sample arrives. From the time the blood is drawn until the family doctor has a negative result, only 1 to 2 working days pass due to the automation that is now widespread. If the patient goes straight to a medical laboratory, the result can often be picked up on the same day. Processing is only extended if the results of the screening test are noticeable, as confirmation tests are then required.

The immunassays up to and including the 3rd generation can only detect antibodies against HIV-1 and HIV-2, but not the virus itself. Since the body needs several weeks for the production of these antibodies as part of an immune response , it is only possible to detect them twelve weeks of possible infection with HIV, assume that the test will be positive in almost all infected people. This period between possible infection and reliable detection of the antibodies is known as the “ diagnostic gap ”. This generation of tests is therefore hardly in routine use anymore.

4th generation immunassays (since 1999) can also detect the so-called p24 antigen in the blood, a protein that is in the envelope ( capsid ) of HIV-1. This is already found in the blood before the HIV antibodies are formed, which shortens the diagnostic gap with regard to HIV-1.

Immunassays have a limited specificity, which means that false-positive results are obtained in some test subjects. Therefore, if the result of the immunoassay is positive, a confirmation test using the more complex Western blot method (more precisely immunoblot) is carried out. Only if this is also positive will the patient be informed of the “HIV-positive” result.

Technical implementation of an immunassay (in the laboratory)

For frequently requested tests, such as for HIV or hepatitis, fully automated laboratory machines (so-called random access analyzers) are now mainly used in order to be able to measure individual samples over the entire day. The classic ELISA using microtiter plates, which is designed to carry out measurement series, is only used occasionally in HIV diagnostics because of the urgency for the patient.

The blood cells are separated from the blood sample to be tested by centrifugation and the remaining yellowish-clear liquid, the so-called blood serum , is barcoded into the laboratory machine, which carries out the test fully automatically and transfers the result to the laboratory IT system. The pure measuring process ranges from minutes (laboratory machine) to hours (ELISA).

If there are antibodies in the serum that were produced by the immune system of an HIV-infected person, they will attach to the inactivated HIV proteins contained in the test kit. Depending on the detection principle of the immunassay used, the subsequent evaluation of the measurement signal, e.g. B. by fluorescence , chemiluminescence or enzymatically . The test is always evaluated in comparison to HIV-positive and HIV-negative control samples.

Benefits of the immunassay

Immunassays are a relatively inexpensive, simple and, above all, quick way of detecting the HI virus. The test at the family doctor costs around 20-25 euros as a self-payer service (fee schedule for doctors 2010), although many health authorities also offer such tests more cheaply or free of charge (but usually not until 12 weeks after risk exposure). If the immunassay is carried out sufficiently after the last risk exposure, the results are extremely accurate.

Disadvantages of the immunassay

Time course of seroconversion rates in the presence of HIV infection (proportion of infected patients who have already developed antibodies)

The greatest disadvantage of the immunassay is that an almost 100% probability of excluding an HIV infection is usually only given after twelve weeks. This means that immunassays that are carried out before the end of the twelve-week period will not recognize any existing HIV infection because not enough HIV antibodies have yet been produced in the human organism. In the case of the immunassay, the “diagnostic gap” (period from the time of infection to reliable detection of the infection) is therefore relatively large. This applies in particular to HIV-2, since in this species the p26 antigen occurs instead of the p24 antigen (which could be detected using 4th generation immunassays) (which cannot be detected using 4th generation immunassays). However, HIV-2 is mostly restricted to West Africa and is extremely rare in Germany (an average of 1–2 cases per year).

The following detection probabilities apply when using immunassays (for infected people):

  • After 4 weeks, the test gives a correct result in 60 to 65% of cases,
  • after 6 weeks in 80%,
  • after 8 weeks in 90%,
  • after 12 weeks in 95% (other sources speak of 99%).

According to the Robert Koch Institute , an infection can be ruled out with great certainty after three months with a negative antibody test.

Second diagnostic gap possible in 4th generation immunassays

In order to shorten the diagnostic gap between risk contact and detection of infection, the immunassays have been further developed in such a way that tests of the 4th generation now also contain HIV-1 p24 antigen detection in addition to pure antibody detection. Since the HIV-1 p24 antigen can already be detected at a point in time at which a pure antibody test would not work, an HIV-1 infection can usually be determined earlier. It must be noted, however, that the HIV-1 p24 antigen is only detectable in the body for about four weeks. This is normally not disadvantageous, since the detectability of the HIV-1 p24 antigen and the detectability of HIV antibodies overlap in time, so that the combination tests (4th generation immunassay) either focus on one or the other or but respond to both components. In individual cases, however, it has happened that the p24 antigen had already fallen below the detection limit of the 4th generation of HIV tests, while the HIV antibodies had not yet exceeded the detection limit. A second diagnostic gap had thus opened up, i.e. a period at which the HIV-1 p24 antigen was no longer detectable, but the HIV antibodies were not yet detectable. If a blood sample was taken during this time and tested using an immunassay (4th generation), a false-negative result was determined in these patients. The test manufacturers have already made efforts to close the “second diagnostic gap”.

Immunoassay Accuracy

The sensitivity and specificity of the immunassay vary depending on the test kit used. Sensitivities of 99.9% are sometimes given. This would mean that out of 1000 HIV-positive patients 999 will be recognized as such and one will receive a false negative result. The specificity is sometimes up to 99.9%. (Thought model to clarify: If each of the 80 million inhabitants of Germany were to be tested, 80,000 people would get a false-positive result using the ELISA test alone ; for comparison, around 78,000 are actually infected with HIV). The positive predictive value , i.e. the probability that a person with a positive test is actually infected, depends on the prevalence in the group tested and can therefore not be given in general. If the prevalence is low, such as B. in people without risk factors, it is below 50%, well below the specificity. If, on the other hand, there are risk factors, the value increases rapidly and reaches values ​​close to the specificity (for the evaluation of a test result, see also: Evaluation of a classifier ). However, false positive reports to patients are almost completely avoided by the Western blot confirmation test described below.

Miscellaneous about the immunassay

There are immunassays for the detection of individual HIV antigens, e.g. B. p24 antigen. These only play a role for special questions.

Western blot confirmation test

If the immunassay is positive or borderline and thus there is a first indication of an existing HIV infection, the laboratory will routinely carry out a serological confirmation test. This confirmation test is carried out using the more complex Western blot method (more precisely immunoblot), which also detects HIV-specific antibodies. For this purpose, the manufacturer of the test applies a number of different HIV proteins side by side to a carrier membrane. The strip is placed in a diluted serum sample from the patient. If antibodies to HIV are present in the patient, they will attach to the virus proteins on the strip. After further work steps, dark lines become visible on the test strip. They show which virus proteins the person has raised antibodies against. Immunoblots, in which virus proteins from HIV-1 and HIV-2 are represented, can also enable a distinction between the two virus types. According to WHO recommendations, the diagnosis “HIV-positive” is based on the detection of antibodies against at least two different virus proteins. In this way, the previously positive or borderline search test is refuted or confirmed and an even greater specificity (approx. 99.9996%) is achieved.

If a positive result in the immunassay cannot be confirmed using Western Blot (which can only detect antibodies), HIV-PCR must be carried out for direct pathogen detection, because the immunassay may only react to the p24 antigen of the virus while no antibodies are present yet. If the immunassay can neither be confirmed by blot nor by HIV-PCR , it can be assumed that the immunassay reacted “unspecifically”, ie. H. became positive for anything other than HIV infection. Antibody tests can give false positive results after recent acute illnesses, flu vaccinations and allergies. In such cases, the person may get negative results months after a positive immunassay.

The patient is only informed of a positive result if both the screening test and the Western Blot are positive, i.e. H. antibodies are definitely detectable, or if the virus can be detected directly in the patient using PCR.

RT-PCR test

How the PCR test works

The RT-PCR method is one of the most precise methods of testing for HIV infections, but it is also one of the most expensive. Just like in the 4th generation immunassay, where the viruses can be detected directly using the p24 antigen, in the PCR test direct detection is carried out by checking for specifically viral nucleic acid sequences. In the first step, viral RNA is converted into cDNA by reverse transcriptase (RT) . This cDNA is then replicated as part of the polymerase chain reaction (PCR). Since the replication is exponential, quantitative PCR tests also allow statements to be made about the original virus concentration (viral load) of the sample, whereby the measuring range of 20 or 40 copies / ml covered by the commercial laboratory tests (e.g. Roche Diagnostics COBAS TaqMan or Abbott RealTime) up to 10,000,000 copies / ml blood. If such a test turns out negative, either there is no HIV infection or the viral load is below the detection limit. As there is an extraordinarily high viral load in the initial phase of an HIV infection due to the lack of the body's own antibodies, a negative PCR test carried out at least 15 days after the risk exposure is usually a clear sign of the absence of HIV -To evaluate infection.

The PCR test takes several hours. As a rule, one to two or three to four working days pass from the time the blood is taken to the transmission of the results, depending on whether the blood was taken directly in the laboratory or at the treating doctor. The PCR method is used as standard in infected people to monitor antiretroviral therapy and is also used in blood donation. In addition, if an acute HIV infection is suspected, the PCR test is carried out by default upon admission to the rescue center.

PCR tests accompanying antiretroviral therapy

In order to be able to monitor the progress and success of the antiretroviral therapy , a quantitative PCR method is used, which determines the number of viral RNA copies in the blood. These blood tests are usually done every three months, which allows you to track the development of the viral load in the infected person's blood. This is necessary, among other things, to monitor the success of antiretroviral therapy, the aim of which is to reduce the viral load until it is permanently below the detection limit.

PCR tests in the blood donation system

For blood and plasma donations, PCR tests are mandatory in order to weed out positive donations. For this purpose, blood donation samples from dozens of people are usually mixed (technical term pooling ) and then jointly examined for HIV genetic material. If the result is positive, the blood samples from smaller and even smaller subgroups are examined in order to trace the origin of the virus genome found back to the individual donor.

PCR test for diagnosis

For the primary diagnosis of an HIV infection, quantitative, but qualitative PCR tests should be used, as these are methodologically simpler and can therefore also respond to HIV subtypes and strains for which a quantitative PCR test may be negative. The costs of around 100–150 euros often have to be borne by yourself. Due to the very low prevalence of HIV-2 in Central Europe (see section “Disadvantages”), an HIV-1 PCR test is usually sufficient - unless the donor had a known HIV-2 infection or was exposed to risk held in West Africa. In contrast to the antibody test, the qualitative PCR test can be carried out as early as 15 days after the risk exposure, because the direct detection of the viral RNA ensures a reliable detection probability at this point in time. However, the manufacturers expressly do not recommend the use of the PCR test to rule out an HIV infection, and the Robert Koch Institute also rejects the use of PCR for diagnostic purposes.

Advantages of the PCR / RT-PCR method

In contrast to the ELISA method, the PCR / RT-PCR method has a smaller diagnostic gap, as a statement about a possible HIV infection can be made just 15 days after a risk contact. The PCR method has a very high specificity of 100% and the best sensitivity of all HIV tests with a lower detection limit of 40 copies / ml (95% sensitivity), which allows almost 100% detection from 75 copies / ml. The advantage of early detection of an HIV infection lies in the presumed positive effect that is ascribed to the start of treatment during the acute phase of HIV infection.

Disadvantages of the PCR / RT-PCR method

A voluntary HIV test using the PCR / RT-PCR method must be paid for by the patient. The costs are around 100 to 150 euros per HIV type (as of 2008). In addition to HIV-1, which is widespread in Germany with around 99.7% of all HIV infections here, tests can also be carried out for the much rarer HIV-2 (only 100 confirmed cases in Germany, as of October 2007).

HIV testing in newborns

In the case of newborns, an antibody test has no informative value, as the mother's IgG antibodies pass through the placenta into the child's blood and therefore produce a false positive test result. Commercially available tests for the detection of IgM or IgA antibodies, which are formed by the child, are not yet available. Therefore, the common method of examination for newborns and infants is RT-PCR.

Point of Care Tests

In certain situations, a quick diagnosis of an existing HIV infection can be useful. An example of this is inadvertent blood contact during a surgical procedure, for example when the operating doctor pricks a needle and the patient's HIV status is unknown. If the patient is found to be infected with HIV, the doctor can initiate HIV post- exposure prophylaxis, the success of which depends largely on how short the period between exposure and the start of prophylaxis is. The rapid test delivers a result after 15-30 minutes, although the risk of a false positive result is higher than with other test methods. If the result is borderline or positive, another test (usually Western blot) must always be carried out to confirm before the diagnosis “HIV-positive” can be made.

Such test procedures are called "point-of-care" tests , home tests , rapid tests or "bedside" tests. They are based on one of the four immunodiagnostic principles of particle agglutination, immunodot (dipstick), immunofiltration or immunochromatography.

The background to the development of point-of-care tests was the need to create a way of quickly detecting HIV infections, even in countries with poor medical infrastructure, such as large parts of Africa. The UN is the largest client of point-of-care tests and distributes them millions of times in African countries.

Technical implementation of a point-of-care test

The patient pricks a fingertip or an earlobe using a mechanical prick or a lancet included in the test set . Now the patient can drop a drop of blood into the sample well of the test plate and add a drop of buffer solution. After the specified time, the display of the test plate is checked. If only a rash appears in the control area, the test was carried out correctly and no HIV infection has (yet) been detected. If a rash appears in the control area as well as in the test area, the test was carried out correctly and the test is positive.

Benefits of Point of Care Testing

In many third world countries, point-of-care tests are the only way to provide evidence for large parts of the population. Furthermore, due to the fast response time (15 min.), In contrast to the classic test in the laboratory, there is no waiting for the laboratory results. Point-of-care tests analogous to the Generation 4 ELISA test have also been available since 2010. In addition to searching for antibodies, the HIV-1 p24 antigen is also searched for, which can be detected by the test 15 days to 45 days at the latest after infection.

Disadvantages of point-of-care testing

Point-of-care tests are often carried out in-house by the potentially infected person without the presence of a laboratory qualified person. In addition to possible errors in the execution of the test, the patient is the only one faced with the possibly positive test result. In such a situation there is a risk that someone who has tested positive will commit a short-circuit act (e.g. suicide) because there is no one there to give him expert advice. Since confirmatory testing is often not performed, there are additional dangers. A false negative result can lead to careless behavior and thus to the infection of other people.

Since most point-of-care tests only aim to detect antibodies, an interval of at least three months is required - analogous to a 3rd generation ELISA test - in order to make an almost one hundred percent statement regarding the exclusion of an HIV infection to be able to. Current point-of-care tests also detect antibodies against HIV-2. Only one of the point-of-care tests also looks for the HIV-p24 antigen (see ELISA test generation 4).

In Germany, medical experts used to reject the free sale of HIV home tests as a matter of principle, as such tests do not meet the globally recognized "VCT standards" (Voluntary Counseling and Testing), according to which HIV tests are only carried out voluntarily and in the context of a counseling situation should. And finally, in contrast to the quality-assured use of rapid tests in the healthcare sector, no corresponding quality control is possible for home tests.

Legal situation in Germany

HIV home tests have been freely available in Germany since September 29, 2018.

Deutsche Aidshilfe announced in September 2019 that around 30,000 tests had been taken in the first year.

Reporting requirement

In Germany, according to the Infection Protection Act ( Section 7 IfSG), the diagnostic laboratory is required to anonymously report an HIV infection to the Robert Koch Institute in Berlin. However, there is no obligation to report the full AIDS picture . Conversely, an HIV infection is not notifiable in Austria, but an AIDS disease is. This is done anonymously to the district administrative authority (health department), which forwards the data to an electronic register of the Ministry of Health, Family and Youth.

literature

  • Lutz Gürtler: Retroviruses . In: Birgid Neumeister, Heinrich K. Geiss, Rüdiger W. Braun, Peter Kimmig (eds.): Microbiological diagnostics . 2nd Edition. Georg Thieme Verlag, Stuttgart / New York 2009, ISBN 978-3-13-743602-7 , p. 959-969 .

Web links

Wiktionary: HIV test  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ↑ a b c Lutz Gürtler: Retroviruses . In: Birgid Neumeister, Heinrich K. Geiss, Rüdiger W. Braun and Peter Kimmig (eds.): Microbiological diagnostics . 2nd Edition. Georg Thieme Verlag, Stuttgart / New York 2009, ISBN 978-3-13-743602-7 , p. 959-969 .
  2. ↑ hivandmore.de Nationwide overview of anonymous HIV test centers with addresses, opening times and waiting times for the result
  3. ↑ HIV self-tests , website of the Paul Ehrlich Institute , accessed on November 25, 2018.
  4. ↑ Run on HIV self-tests in pharmacies , DAZ.online, July 30, 2018.
  5. ↑ a b c d e f hivandmore
  6. ↑ macht-doch-jeder.de
  7. ↑ Information folder for advice in Aidshilfen 2016
  8. ↑ BZgA 2007
  9. ^ Health advisor Berlin
  10. ↑ Medical Tribune, according to Spiegel 17/1988 of 25 April 1988
  11. ↑ HIV-2 - In Big Brother's Shadow . Retrieved May 23, 2014.
  12. ↑ a b FAQ of the Robert Koch Institute . Status: November 26, 2013. Accessed July 7, 2014.
  13. ↑ HIV / AIDS - RKI guide for doctors . Archived from the original on March 11, 2014. Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. Retrieved March 10, 2014. @1@ 2Template: Webachiv / IABot / edoc.rki.de
  14. ^ T. Meier, E. Knoll, M. Henkes, G. Enders, R. Braun: Evidence for a diagnostic window in fourth generation assays for HIV. In: Journal of clinical virology: the official publication of the Pan American Society for Clinical Virology. Volume 23, Numbers 1-2, December 2001, pp. 113-116, ISSN  1386-6532 . PMID 11595590 .
  15. ↑ a b B. Serhir et al .: Multicenter Evaluation of Three Novel 4th Generation HIV Ag / Ab Combo Assays: Abbott Architect, Roche HIV Combi and Siemens Advia Centaur. . In: Sexually Transmitted Infections . .
  16. ↑ Kleinman et al .: False-Positive HIV-1 Test Results in a Low-Risk Screening Setting of Voluntary Blood Donation . In: JAMA . 280, No. 12, 1998, pp. 1080-1085. PMID 9757856 .
  17. ↑ Simonsen et al .: Multiple false reactions in viral antibody screening assays after influenza vaccination. In: American Journal of Epidemiology. Vol. 141, No. 11, pp. 1089-1096.
  18. ↑ RealTime PCR: HIV-1 Assay. Abbott Molecular, accessed July 7, 2014 (English): "This assay is not intended to be used as a donor screening test for HIV-1 or as a diagnostic test to confirm the presence of HIV-1 infection."
  19. ↑ COBAS® AmpliPrep / COBAS® TaqMan® HIV-1 Test, v2.0 (Product sheet). Abbott Molecular, accessed July 7, 2014 : "The COBAS® AmpliPrep / COBAS® TaqMan® HIV-1 Test, v2.0 is not intended for use as a screening test for the presence of HIV-1 in blood or blood products or as a diagnostic test to confirm the presence of HIV-1 infection. "
  20. ↑ Ruby Phelps et al .: Window-period human immunodeficiency virus transmission to two recipients by an adolescent blood donor . In: transfusion . Vol. 44, No. 6, 2004, pp. 929-933, ISSN  0041-1132 .
  21. ↑ COBAS AmpliPrep, COBAS TaqMan HIV-1 test. The fully automated determination of the HIV-1 viral load. Roche Diagnostics, Mannheim, 2005.
  22. ↑ Epidemiological Bulletin . HIV infections and AIDS diseases in Germany . Robert Koch Institute, Berlin 2007.
  23. ^ Draft to amend the German Medical Devices Act of December 18, 2008
  24. ↑ 30,000 tests in one year , Pharmazeutische Zeitung, September 23, 2019.
  25. ↑ Entire legal regulation for the Epidemic Act 1950, version of 07/07/2014. Federal Chancellery, July 7, 2014, accessed on July 7, 2014 .