Medical Microbiology

RNAviruses 453

Toxoplasmaorpapovaviruses(PML,seep. 41 5),orlymphomas.Table8. 4 pre-

sentstheCDC(CentersforDiseaseControl)classificationofthestagesinthe

courseofHIVinfection.ThenumberofCD4+Tcellsandtheoccurrenceofso-

calledAIDS-definingdiseasesdeterminewhetheranHIV-positivepatientis

categorizedasacaseofAIDS(Table8. 4 ).TheprobabilitythatanAIDS-defin-

ingdiseasewilloccurrisesprecipitouslyatCD4+cellcountsbelow 20 0.

Laboratorydiagnosis.Thefollowingdiagnostictoolsarecurrentlyavailable

forconfirmationofanHIVinfection(notthesameasmanifestAIDS,see

above):

&HIVantibodydetection.EIAscreeningtestsarenowavailableusing

geneticallyengineeredorsynthesizedviralantigens(firsttothirdgeneration

ofscreeningtests).Everypositiveresultrequiresconfirmationbyanalterna-

tivetest(Westernblot,seep. 123 andFig.2. 24 ,p. 1 25,p24antigendetection).

Thefourth-generationscreeningtestssimultaneouslydetectantibodiesto

HIV 1 and 2 andp24antigen(combinationtest)andarethuscapableof

detectingprimaryinfectionsthatarestillantibody-negative.

&HIVantigendetection.Inthistest,aviralproteinisdetectedinserum,

usuallycapsidproteinp24.Thep24antigenisdetectableinserumasearly

astwoweeksafterinfectionanddisappearsagainaftereightto 12 weeks.

Followingaclinicallystablelatencyperiod,HIVantigencanbecomedetect-

ablemonthsoryearslater(transitoryorpersistent).Thisrenewedappear-

anceofHIVantigenisusuallyfollowedbymanifestAIDSandistherefore

anegativeprognosticsign.

&RapidHIVtest.Antibody-basedtestsareavailableforrapiddiagnosisin

medicalpractices,hospitals,andhealthcenters.Theirspecificationsare

equivalenttothethird-generationscreeningtests.

&PCR.Themostimportantapplicationofthepolymerasechainreaction

(PCR,seep.409)todayistodeterminetheso-calledviralload,wherebya

commerciallyavailablequantitativeRT-PCR(reversetranscriptasePCR)is

usedtodeterminethenumberofviralRNAmoleculespermlofblood,taking

intoaccounttheaddedstandardamountsofHIVRNA(quantificationstan-

dard).Thistestprovidesaprognosticestimateofhowgreattheriskofpro-

gressiontoAIDSis(manifestationofanAIDS-definingdisease).Itcanalsobe

usedtomonitorthesuccessoftherapywithRTandproteaseinhibitors.

ThefollowingHIVdiagnosticprocedureisnowrecommended:anHIVanti-

bodyscreeningtestshouldfirstbeperformedtodiagnoseanHIVinfection.If

thetestresultispositive,asecondserumspecimenshouldbetestedtocon-

firmtheresultandexcludeconfusionofsera.Iftheinitialscreeningtestis

negative,buta(primary)HIVinfectionisjustifiablysuspected,HIVantigen

canbetested,forinstanceusingthecombinationtest.

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Kayser, Medical Microbiology © 2005 Thieme