Medical Microbiology

LaboratoryDiagnosis 209

Materialfromtherespiratorytract:

— Swabsmearfromtonsils.

— Sinusflushingfluid.

— Pulmonarysecretion.Expectoratedsputumisusuallycontaminatedwith

salivaandthefloraoftheoropharynx.Sincethesecontaminationsinclude

pathogensthatmaycauseinfectionsofthelowerrespiratorytractorgans,

thevalueofpositivefindingswouldbelimited.Thematerialcanbecon-

sideredunsuitablefordiagnostictestingifmorethan 25 oralepitheliaare

presentperviewingframeat 100 !magnification.Morningsputumfrom

flushingthemouthorafterinductionwillresultinsuitablesamples.Spu-

tumisnotanalyzedforanaerobes.

— Usefulalternativestoexpectoratedsputumincludebronchoscopically

sampledbronchialsecretion,flushingfluidfrombronchoalveolarlavage

(BAL),transtrachealaspirateorapulmonarypuncturebiopsy.Thesetypes

ofmaterialarerequiredifananaerobeinfectionissuspected.Thematerial

mustthenbetransportedinspecialanaerobetransportcontainers.

Table3. 6 2! 2 Table:ExplanationandCalculationofSensitivity,Specificity,

andPredictiveValue(Positive–Negative)

Collective Testpositive Testnegative

Infected Correctpositive

cp

Falsenegative

fn

Noninfected Falsepositive

fp

Correctnegative

cn

&Sensitivity(%)measuresthefrequencyofcorrectpositiveresultsintheinfected

collective(horizontaladdition).

&Specificity(%)measuresthefrequencyofcorrectnegativeresultsinthe

noninfectedcollective(horizontaladdition).

Sensitivityð%Þ¼ cp

cpþfn

! 100 ;Specificityð%Þ¼ cn

fpþcn

! 100

&Thepredictivevalueofthepositiveresultexpressestheprobabilitythatapositive

resultindicatesaninfection.Itanalyzesthepositivetestresultsbothinthe

infectedcollectiveandinthenoninfectedcollective(verticaladdition).

&Thepredictivevalueofthenegativeresultexpressestheprobabilitythata

negativeresultindicatesnoninfection.

Pos:pred:valueð%Þ¼

cp

cpþfp!100; Neg:pred:valueð%Þ¼

cn

cnþfn!^100

3

Kayser, Medical Microbiology © 2005 Thieme