Dr Anthony Kelleher
Department of Immunopathology

Monitoring of HIV infection

Monitoring HIV Infection

Plasma Viral load

Plasma viral load is expressed as RNA copies/ml.  Currently, the routinely conducted assay has a lower limit of detection of 50 copies/ml and an upper limit of detection of 100,000 copies/ml. If the viral load is greater than 100,000 copies/ml the laboratory will perform a second test which has an upper limit of detection of 750,000 copies /ml. During Primary infection, peak viral loads are usually in excess of 10⁶ copies /ml and are often in excess of 10⁸ copies/ml.  During the asymptomatic phase of infection viral loads stablise at levels that vary markedly between individuals but are usually in the range of 10⁴  - 10⁵ copies/ml.   With disease progression viral load often rises to greater than 10⁶ copies/ml. The large dynamic range of the test has meant that results are also expressed as the log₁₀ of the copy number (for example 10,000 copies/ml is also expressed as 4.00 log₁₀ copies/ml).  The test depends on massive amplification of viral plasma RNA.  This amplification unavoidably limits the reproducibility of the test.  This means that changes of less than 3 fold or  0.5 log₁₀ copies/ml may be due to test to test variation rather than having clinical implications.  Therefore, the test should be repeated if there is uncertainty regarding its clinical implications.

Following therapy the aim is to reduce viral load as far as possible.   Generally, the aim is to drive the viral load to <50 copies/ml.  This indicates that viral turn over is significantly limited and the ability of the virus to develop drug resistant mutants is greatly curtailed.   

CD4+ T cell counts

CD4+ T cell counts are reported as percentage of total lymphocytes and as absolute number of cells/µL of whole blood. Usually, the absolute count rather than the percentage is used to make clinical decisions.   The normal level of CD4+ T cells is >500 cells/µl. Current recommendations suggest that antiretroviral therapy commence at 350cells/µl. CD4 T cell counts below 200 cells/µl indicate severe immuno-compromise and susceptibility to opportunistic infections.  Chemo-prophylaxis for opportunistic infections should commence when CD4+ T cell counts approach this level.   CD4+ counts less than 100cells/µl are associated with increasing susceptibility to a range of opportunistic infections particularly recurrence of previously quiescent CMV and Toxoplasmosis infections.

Like lymphocyte counts, CD4+ T cell counts will vary with time.   In general, changes in CD4+ T cell counts >30 cells/µL are clinically significant. Usually the % CD4 varies less than the total count.  In individuals who are hypo or asplenic then the % CD4 count should be monitored as the peripheral lymphocyte count will be spuriously raised. 

Post initiation of therapy CD4+ T cell counts usually rise.   Effective therapy usually results in an initial rapid rise in the CD4+ T cell count followed by  a stabilisation or more gradual rise in CD4 count that persists over several months or even years if therapy is effective.

Indications

CD4+ T cell counts and viral loads should be used to monitor established HIV infection and the response to anti-retroviral therapy.  Normally these tests are performed together 3-4 times per year depending on clinical indications.  CD4+ T cell counts and viral load should be performed immediately prior to commencing therapy in order to establish an “off treatment” baseline.  A significant fall in viral load should be seen within 4 weeks of commencing therapy and a viral load less than 50 copies/ml should be attained within 16-20 weeks of therapy. Once stable on a therapeutic regimen then CD4 counts and viral loads can be performed 3-4 time/year unless there is a change in clinical status.

Supplemental Tests

A Full blood count must be ordered in conjunction with a request for CD4+ T cells.  Once the diagnosis is established it is not necessary to repeat the serological tests  performed for the diagnosis of HIV (western blot and ELISAs). In established infection the p24 antigen test provides less information than the viral load and should not be ordered.  When a CD4 count is ordered the results will include a CD8 and total T cell count.  The CD8 T cell count does not usually provide any prognostic information.  Measures of immune activation such as ?2-microglobulin and neopterin or CD8+ CD38+ cell counts do not provide extra clinical information except in particular circumstances.  A failure of response to initiation of therapy or a failure of an established therapeutic regimen requires further investigation. Once compliance with the regimen has been established, tests such as HIV genotypic drug resistance and serum drug levels should be considered, but only ordered after specialist consultation.