Evidence of infection by HIV can be detected in three ways: (1) virus isolation, (2) serologic determination of antiviral antibodies, and (3) measurement of viral nucleic acid or antigens.

A. Virus Isolation

 HIV can be cultured from lymphocytes in peripheral blood (and occasionally from specimens from other sites). The numbers of circulating infected cells vary with the stage of disease (Figure 1). Higher titers of virus are found in the plasma and in peripheral blood cells of patients with AIDS as compared with asymptomatic individuals. The magnitude of plasma viremia appears to be a better correlate of the clinical stage of HIV infection than the presence of any antibodies (Figure 2). The most sensitive virus isolation technique is to cocultivate the test sample with uninfected, mitogen-stimulated peripheral blood mononuclear cells. Primary isolates of HIV grow very slowly compared with laboratory-adapted strains. Viral growth is detected by testing culture supernatant fluids after about 7–14 days for viral reverse transcriptase activity or for virus-specific antigens (p24).

Fig1. Typical course of untreated HIV infection. During the early period after primary infection, there is widespread dissemination of virus and a sharp decrease in the number of CD4 T cells in peripheral blood. An immune response to HIV ensues, with a decrease in detectable viremia followed by a prolonged period of clinical latency. Sensitive assays for viral RNA show that virus is present in the plasma at all times. The CD4 T-cell count continues to decrease during the following years until it reaches a critical level below which there is a substantial risk of opportunistic diseases. (Reproduced with permission from Fauci AS, Lane HC: Human immunodeficiency virus disease: AIDS and related disorders. In Longo DL, Fauci AS, Kasper DL, et al (editors). Harrison’s Principles of Internal Medicine, 18th ed. McGraw-Hill, 2012. © McGraw-Hill Education.)

Fig2. Pattern of HIV antibody responses related to the course of HIV infection. (CTL, cytotoxic T lymphocytes.) (Reproduced with permission from Weiss RA: How does HIV cause AIDS? Science 1993;260:1273. Reprinted with permission from AAAS.)

The vast majority of HIV-1 antibody-positive persons will have virus that can be cultured from their blood cells. However, virus isolation techniques are time consuming and laborious and are limited to research studies. PCR amplification techniques are commonly used for detection of virus in clinical specimens.

B. Serology

 Test kits are commercially available for measuring antibodies by enzyme-linked immunoassay (EIA). If properly per formed, these tests have a sensitivity and specificity exceeding 98%. When EIA-based antibody tests are used for screening populations with a low prevalence of HIV infections (eg, blood donors), a positive test in a serum sample must be confirmed by a repeat test. If the repeat EIA test is reactive, a confirmation test is performed to rule out false-positive EIA results. The most widely used confirmation assay is the West ern blot technique, in which antibodies to HIV proteins of specific molecular weights can be detected. A positive result is defined as the presence of any two bands corresponding to p24, gp41, and gp120/160. The Western blot can be indeterminate or negative early in HIV infection, and detection of HIV RNA is an alternate means for confirmation of the diagnosis. Infection with HIV-2 can yield indeterminate Western blot results for HIV-1 and requires a separate HIV-2 Western blot for confirmation.

The response pattern against specific viral antigens changes over time as patients progress to AIDS. Antibodies to the envelope glycoproteins (gp41, gp120, gp160) are maintained, but those directed against the Gag proteins (p17, p24, p55) decline. The decline of anti-p24 may herald the beginning of clinical signs and other immunologic markers of progression (Figure 2).

Simple, rapid tests for detecting HIV antibodies are available for use in laboratories ill-equipped to perform EIA tests and in settings where test results are desired with little delay. The simple tests can be performed on blood or oral fluid and are based on principles such as particle agglutination or immunodot reactions. There are rapid tests that can detect HIV antibodies in whole-blood specimens that require no processing. These tests can be performed outside the traditional laboratory setting. The mean time to seroconversion after HIV infection is 3–4 weeks. Most individuals will have detectable antibodies within 6–12 weeks after infection, whereas virtually all will be positive within 6 months. HIV infection for longer than 6 months without a detectable anti body response is very uncommon.

C. Detection of Viral Nucleic Acid or Antigens

HIV nucleic acid testing (NAT) assays, such as the RT-PCR, DNA PCR, and branched DNA (bDNA) tests, are commonly used to detect viral nucleic acid in clinical specimens. The RT-PCR assay uses an enzymatic method to amplify HIV RNA; the bDNA assay amplifies viral RNA by sequential oligonucleotide hybridization steps. These molecular-based tests are very sensitive and form the basis for plasma viral load determinations. The HIV RNA levels are important predictive markers of disease progression and valuable tools with which to monitor the effectiveness of antiviral therapies. Dried blood spot specimens are an alternative to plasma specimens for viral monitoring in resource-limited settings.

Early diagnosis of HIV infection in infants born to infected mothers can be accomplished using plasma HIV-1 RNA or whole-blood DNA PCR to detect chromosomally integrated (proviral) DNA. The presence of maternal anti bodies makes serologic tests uninformative.

Low levels of circulating HIV-1 p24 antigen can be detected in the plasma by EIA soon after infection. The antigen often becomes undetectable after antibodies develop (because the p24 protein is complexed with p24 antibodies) but may reappear late in the course of infection, indicating a poor prognosis. Fourth-generation HIV diagnostic assays that include HIV antibody and p24 antigen detection can decrease the window period when earlier serologic tests would not detect infection. Newer fifth-generation tests simultaneously detect and differentiate HIV-1 antibody, HIV-2 antibody, and p24 anti gen. Detection of p24 antigen in HIV antibody-negative individuals is important because individuals in this stage are highly viremic and can readily transmit infection. HIV NAT further reduces the infectious window period and is commonly performed on patients with suspected acute HIV infection, health care workers exposed through needlestick injuries, and blood donors.

D. HIV Resistance Testing

HIV genotyping is the most common method to determine viral resistance. It is performed by sequencing portions of the reverse transcriptase and protease genes to identify mutations known to confer resistance to inhibitors of these gene products. Mutations are identified as promoting resistance if they allow virus to be grown in the presence of drug, or are associated with clinical treatment failures. Databases maintained by the International AIDS Society and Stanford University are updated with newly identified resistance mutations. Development of an optimal treatment regimen is complicated, requiring knowledge of viral resistance patterns, drug activities, side effects, and interactions and typically requires a specialist in HIV treatment.

Assays are also available to assess HIV integrase and fusion inhibitor resistance. Coreceptor tropism is a phenotypic assay to determine whether virus is likely to respond to CCR5 antagonist drugs.

Phenotypic resistance tests involve growth of recombinant virus in the presence of antiviral drugs. The relevant genes (reverse transcriptase, protease, or integrase) are cloned from the patient’s virus into a laboratory HIV strain and the concentration of drug that inhibits 50% of viral replication (IC₅₀ ) is determined. The ratio of patient virus IC₅₀ to the reference IC50 value indicates the fold resistance to the tested drug.

HIV resistance testing is recommended at the time of initial diagnosis and when managing treatment failures or suboptimal viral load reduction. Genotypic resistance testing is the standard method, but phenotypic testing can be useful in patients with complex resistance mutation patterns.

اخر الاخبار

اخبار العتبة العباسية المقدسة

العتبة العباسية المقدسة: نهج البلاغة مصدر صافٍ للفكر والمعرفة ومحصِّنٌ من الشبهات

العتبة العباسية المقدسة تطلق محاضرات محفل نهج البلاغة الأسبوعي في ذي قار

العتبتان المقدستان الحسينية والعباسية تدعوان للمشاركة في مهرجان الشموع بنسخته السادسة والعشرين

بالتعاون مع مستشفى الكفيل.. قسم الشؤون الطبّية يواصل إقامة البرنامج التدريبي لملاكاته النسوية

المزيد

الآخبار الصحية

الصحة العالمية تكشف حقيقة انتشار فيروس نيباه خارج الهند

متى يصبح محيط الخصر "مؤشر خطر"؟

دراسة تكشف دور "العوامل الوراثية" في تحديد عمر الإنسان

رائحة كريهة في الجسم.. "علامة خفية" أخطر مما تتصور

المزيد

الآخبار التكنلوجية

مايكروسوفت تكشف عن الجيل الثاني من شرائحها للذكاء الاصطناعي

خرافة "ثمانية أكواب يوميا".. كم من الماء يحتاج جسمك فعليا؟

هل توجد حياة خارج كوكبنا؟.. اكتشاف جديد يثير اهتمام العلماء

اكتشاف الخلايا المسؤولة عن فقدان ذاكرة الطفولة

المزيد

مواضيع ذات صلة

HIV Infections in Humans :Clinical Findings

HIV Infections in Humans : Pathogenesis and Pathology

Animal Lentivirus Systems

Classification of Lentiviruses

Viral DNA Is Integrated into the Host-Cell Genome in Some Nonlytic Viral Growth Cycles

Lytic Viral Growth Cycles Lead to Death of Host Cells

Viral Capsids Are Regular Arrays of One or a Few Types of Protein

Structure and Composition of Lentiviruses

HERPESVIRUSES

POLYOMAVIRUSES

Human T-Lymphotropic Viruses

Replication of Retroviruses