Medical Microbiology

LaboratoryDiagnosis 411

copiedwithanaddedpolymerase,wherebytheoligonucleotidesactasprim-

ers.Thenewandoldstrandsareonceagainseparatedbyheatandthe

reactionisstartedoveragain.Runningseveralsuchcyclesamplifiestheorig-

inalviralDNAbyafactorofmanythousands.Beginningwiththesecondgen-

eration,thenewlysynthesizedDNAstrandsshowauniform,definedlength

andarethereforedetectablebymeansofgelelectrophoresis.Thespecificity

ofthereactionisverifiedbycheckingthesequencesoftheseDNAstrandsby

meansofhybridizationorsequencing.Theamplificationanddetectionsys-

temsinusetodayformanyvirusesareincreasinglycommerciallyavailable,

andinsomecasesarealsodesignedtoprovidequantitativedataonthe“viral

load.”

Serodiagnosis

Ifaviralinfectioninduceshumoralimmunity(seep. 4 8f.and 40 1),there-

sultingantibodiescanbeusedinaserodiagnosis.Wheninterpretingtheser-

ologicaldata,oneisconfrontedbytheproblemofdecidingwhethertheob-

servedreactionsindicateafresh,currentinfectionorearliercontactwiththe

virusinquestion.Twocriteriacanhelpwiththisdecision:

DetectionofIgM(withoutIgG)provesthepresenceofafreshprimaryinfec-

tion.IgMisnowusuallydetectedbyspecificserumagainsthumanIgMinthe

so-calledcapturetest,anEIA(p. 1 28).

TotestforIgMalone,abloodspecimenmustbeobtainedveryearlyinthe

infectioncycle.ConcurrentdetectionofIgGandIgMinbloodsampledsome-

whatlaterinthecourseofthediseasewouldalsoindicateafreshinfection.It

could,however,alsoindicateareactivatedlatentinfectionorananamnestic

reaction(i.e.,anonspecificincreaseinantibodiesinreactiontoanonrelated

infection),sinceIgMcanalsobeproducedinbothofthesecases.

AfourfoldincreaseintheIgGtiterwithin 10 – 14 daysearlyoninthe

courseoftheinfectionoradropofthesamedimensionslaterinthecourse

wouldalsobeconfirmation.

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Kayser, Medical Microbiology © 2005 Thieme