J. Pediatr. (Rio J.) vol.91 número6

www.jped.com.br

REVIEW

ARTICLE

Sweat

conductivity

and

coulometric

quantitative

test

in

neonatal

cystic

fibrosis

screening

夽

Mouseline

Torquato

Domingos

_,

_Neiva

_Isabel

_Rodrigues

_Magdalena

_,

Mônica

Nunes

Lima

Cat

_,

_Alexandra

_Mitiru

_Watanabe

_,

_Nelson

_Augusto

_Rosário

_Filho

a_{Servic¸odeReferênciaemTriagemNeonatal(SRTN),Fundac¸ãoEcumênicadeProtec¸ãoaoExcepcional(FEPE),Curitiba,PR,Brazil} b_{DepartmentofPediatrics,UniversidadeFederaldoParaná(UFPR),Curitiba,PR,Brazil}

Received12September2014;accepted11March2015 Availableonline16June2015

KEYWORDS

Cysticfibrosis; Neonatalscreening; Sweattest;

Conductivity; Pilocarpine; Coulometric measurement

Abstract

Objective: Tocomparetheresultsobtainedwiththesweattestusingtheconductivitymethod andcoulometricmeasurementofsweatchlorideinnewborns(NBs)withsuspectedcysticfibrosis (CF)intheneonatalscreeningprogram.

Methods: Thesweattestwasperformedsimultaneouslybybothmethodsinchildrenwithand withoutCF.ThecutoffvaluestoconfirmCFwere>50mmol/Lintheconductivityand>60mmol/L inthecoulometrictest.

Results: Therewere444infantswithoutCF(185males,234females,and24unreported) sub-mittedtothesweattestthroughconductivityandcoulometricmeasurementsimultaneously, obtainingmedianresultsof32mmol/Land12mmol/L,respectively.For90infantswithCF,the medianvaluesofconductivityandcoulometricmeasurementwere108mmol/Land97mmol/L, respectively.Thefalsepositiverateforconductivitywas16.7%,andwashigherthan50mmol/L inallpatientswithCF,whichgivesthismethodasensitivityof100%(95%CI:93.8---97.8), speci-ficityof96.2% (95% CI:93.8---97.8), positive predictive value of83.3% (95%CI: 74.4---91.1), negativepredictivevalue of100%(95%CI:90.5---109.4),and9.8%accuracy. Thecorrelation betweenthemethodswasr=0.97(p>0.001).Thebestsuggestedcutoffvaluewas69.0mmol/L, withakappacoefficient=0.89.

Conclusion: Theconductivitytest showedexcellentcorrelationwiththequantitative coulo-metrictest,highsensitivityandspecificity,andcanbeusedinthediagnosisofCFinchildren detectedthroughnewbornscreening.

©2015SociedadeBrasileiradePediatria.PublishedbyElsevierEditoraLtda.Allrightsreserved.

夽

Pleasecitethisarticleas:DomingosMT,MagdalenaNI,CatMN,WatanabeAM,RosárioFilhoNA.Sweatconductivityandcoulometric quantitativetestinneonatalcysticfibrosisscreening.JPediatr(RioJ).2015;91:590---5.

∗_{Correspondingauthor.}

E-mail:mussetd@hotmail.com(M.T.Domingos).

http://dx.doi.org/10.1016/j.jped.2015.03.003

PALAVRAS-CHAVE

Fibrosecística; Triagemneonatal; Testedosuor; Condutividade; Pilocarpina; Dosagem coulométrica

Condutividadeetestequantitativocoulométriconatriagemneonatalparafibrose cística

Resumo

Objetivo: Compararosresultadosobtidosnotestedosuorpelométododacondutividadeea dosagemcoulométricadecloretonosuoremrecémnascidos(RN)suspeitosdatriagemneonatal parafibrosecística(FC).

Métodos: Otestedosuorfoirealizadosimultaneamentepelosdoismétodosemcrianc¸ascome semFC.OsvaloresdecorteparaconfirmarFCforamnacondutividade>50mmol/Lenoteste coulométrico>60mmol/L.

Resultados: QuatrocentosequarentaequatroRNsemFC(185dosexomasculino,234femininos e 24não informado)realizaramo testedosuor porcondutividadee dosagemcoulométrica simultaneamenteeobtiveramresultadomedianode32mmol/Le12mmol/Lrespectivamente. Para osnoventaRN comFC osvalores medianosde condutividadee dosagemcoulométrica foram108mmol/Le97mmol/Lrespectivamente.Oíndicedefalsopositivoparacondutividade foide16,7%eemtodosospacientesFCfoisuperiora50mmol/Lconferindoaométodo100% de sensibilidade(IC95%=93,8a97,8), especificidadede96,2% (IC95%=93,8a97,8),valor preditivo positivo 83,3(IC95%=74,4a91,1),valor preditivo negativo100%(IC95%=90,5a 109,4)eacurácia9,8%.Acorrelac¸ãoentreosmétodosfoider=0,97(p>0,001).Omelhorvalor decortesugeridofoide69,0mmol/L,coeficientedekappa=0,89.

Conclusão: O teste da condutividade apresentou excelente correlac¸ão com o quantitativo coulométrico,altasensibilidadeeespecificidade,podendoserutilizadonodiagnósticodaFC emcrianc¸asdetectadaspelatriagemneonatal.

©2015SociedadeBrasileiradePediatria.PublicadoporElsevierEditoraLtda.Todososdireitos reservados.

Introduction

Cysticfibrosis(CF)isanautosomalrecessivedisease, result-ing from mutations in the gene located in the long arm of chromosome 7, cystic fibrosis transmembrane conduc-tanceregulator(CFTR),withanincidenceofabout1:10,000 in Brazil.1,2 _{It is a multisystemic disease and pulmonary} involvement accounts for the majority of patient mor-bimortality.Theclinicalsignsandsymptomsincludechronic pulmonarydisease,pancreaticfailure,andabnormalitiesin thegastrointestinaltractandsweat glandsecreting chan-nels, with an increase of chloride concentration in the sweat.3---5 _{Survivaldependsonearlydiagnosisconfirmation} andtreatmentinitiationattheearlystagesofthedisease. Identificationisattainedbythepresenceofsignsor symp-toms;familyhistory;alteredimmunoreactive trypsin(IRT) inneonatalscreeningandconfirmedbypositivesweattestin twoseparatemeasurements,presenceoftwoCFmutations, or altered nasalpotential differencetest. The latter test requiresconsiderableskill,andisrarelyusedinBrazil.6,7

In Brazil, CF screening was introduced in the National

Neonatal Screening Program (PNTN) in 2001 with the

immunoreactive trypsintest (IRT)performed inblood col-lectedfromnewbornsandthesweattest.2

The confirmatorytest, the gold standard, is the quan-titativeanalysisofelectrolytesinsweat,withanaccuracy >90%.8_{Sweatingisstimulatedbypilocarpine,performedby} iontophoresis;sweatisobtainedusingtheGibsonandCooke method.However,skillisnecessarytopreventevaporation duringcollectionandthesubsequentdeterminationofsweat weight on an analytical scale, followed by the accurate chemicalcompositionofthesample.The sweatcollection systemwithacapillarymicrotubeMacroduct®_(WescorInc.,

Logan,UT,USA)hasbeenwidelyusedduetoitssimplicity andefficiency.Themicrotubepreventsevaporation,andthe weighinganddilutionstepsareeliminated.

Sweatcanhave itsioniccompositionanalyzed immedi-ately or be submitted toa conductivity analyzer prior to chemicaldetermination.9_{Thetestispositiveifthechloride} concentration is >60mmol/L in at least two independent measurements.8,10 _{Forinfantsyoungerthan6months,}

val-ues between 30mmol/L and 50mmol/L are considered

suspect.9---11 _{Theconductivitytestalsoconstitutesan} alter-native and valid method for the laboratory diagnosis of CF.7,8,10 _{The system measures the capacity of the sweat} toconductelectricalcurrent inmilliamperes(mAs),which dependsontheconcentrationofNa+_andCl−_.12

The Sweat Check 3120® _{(WescorInc., Logan, UT, USA)}

conductivity analyzer, specifically designed for use with the Macroduct® _{(Wescor Inc., Logan, UT, USA) sweat}

col-lector,measures conductivityin a 6---10␮Lsample. Values >80mmol/L and a compatible clinical setting justify the start of treatment, whereas values between 50mmol/L and 80mmol/L indicate the need to perform the quanti-tativetest,becauseconductivityisconsideredascreening method,asit isnot selectivetothe chloride ion.8---10 _The aimofthisstudywastocomparechloridelevelsinsweatby thequantitativecoulometrictestwithconductivityvaluesin newbornswithandwithoutCF,fromtheneonatalscreening programofthestateofParaná.

Methods

cohort study that evaluated the neonatal screening pro-gram for CF in the state of Paraná. From January 2002 to December 2008, 1,218,642 newborns were evaluated. Initialscreening wasperformed throughIRTmeasurement in blood samples obtained from dermal puncture of the heel, impregnated on filter paper and then analyzed by immunofluorometricassayin a AutoDELFIAsystem (Perkin Elmer®_{,MA,USA),conductedinthreestages.}

Childrenwithalteredvalues≥70mg/Lin twodifferent samplesobtainedduringthefirst30daysoflifewere con-tactedbyateamofsocialworkersthroughactivesearchto besubmittedtoafreesweattest(conductivityfollowedby coulometricquantitativetestforchloridemeasurement)in ordertoruleoutsuspicionforCF.

An informative questionnaire on the tests was applied toparents or guardians,whoafter being recalled, volun-teeredandconsented toitsperformance. Atotalof 2309 infantswere submittedtothetest using bothmethods; a repeatedtestwasrequiredin99infants(4.3%)dueto insuf-ficientquantityofsweat.Ofthe2309children,90casesof CFwereconfirmed.Forthestudy,agroupconsistingof444 infantswasselectedbyconveniencefromthe2219children withnormalsweattest,aswellasthe90caseswithpositive sweattestandconfirmedCF.

Sweatcollection

Sweating was induced by the iontophoresis technique withpilocarpine. Gelatinous discs impregnated with 0.5% PILOGEL® _{(Pilocarpine hydrochloride, Alcon Laboratories,}

UK) were affixed using standard WESCOR® _electrodes

(WescorInc.,Logan,UT,USA),positiveandnegativefitting thechild’sarmperfectlyafterrigorousasepsisandcleaning withdeionizedwater.Thedevicepromotesthestimulation throughaniontophoreticcurrent of1.5mAfor5min. Pilo-carpineis ionized and guided toan areaof skin that has sweatglands,preferablyintheanteriorforearm.8,9_Ifthis sitewasnotadequateforthecollection,thentheanterior thighareawasused.

Afterstimulation,theareawascleanedwithdeionized water and the sweat collected directly in a disposable Macroduct® _{(WescorInc.,Logan,UT,USA)firmlyaffixedto}

thesite,accordingtothemanufacturer’srecommendation. Collectiontimedidnotexceed30min,norwasitinferiorto 20min.9 _{The meanrateduringthecollectionwasnotless} than 1g/m2_{/min or 10}

␮L of sweat using the Macroduct® (WescorInc.,Logan,UT, USA)collectionsystem, the min-imumvolumeofsweatrequiredforthisdevice.9,13

Conductivitymethod

Immediately after collection, the sweat sample was sub-mittedtotheconductivitytest bytheSweatCheck 3120®

device.Thenormal,borderline,andabnormalvalueswere establishedaccordingtotheequationassociating conductiv-itywithchloride.11,13_{Aftertheconductivitywasmeasured,} thesamesweatsample,placedinanEppendorftube,was immediatelysentforthequantitativemeasurementof chlo-ride.

To preventtheconductivityresultfrominfluencingthe quantitativemeasurementofchloride,thetwotechniques

wereperformed by differentteamsthat wereunaware of eachother’sresults.

Quantitativemeasurementofchlorideby coulometry

For this measurement,a digital Labconco® _{chloridometer}

(Labconco®_{,KS, USA)wasutilized, which is adevice that}

performscolorimetrictitrationofchlorideions.Aquantity of10␮Lofsweatisnecessaryforthereactiontooccurwithin

secondsofcontactwiththereagentsolutionandthepairof electrodes,providingadigitalreadoutinmmol/L.14---16

Themethodmonitoringfortheinitialscreeningwas car-ried out according to the internal kit controls, TIRIQAS (France) and CDC (Centers for Disease Control and Pre-vention,GA, USA).Forthequality controlofsweat tests, NaCl commercial standard solutions at concentrations of 40mmol/L, 70mmol/L, and 130mmol/L were used in all experiments, in addition to previously prepared internal NaClstandardsof10mmol/L,50mmol/L,and100mmol/L. Thisstudywasconductedinaninstitutionaccreditedbythe MinistryofHealthforneonatalscreeningforCF.

Athospitaldischarge,theserviceusuallydistributesthe ‘‘informationforparents’’leaflet,whichcontains explana-tionsonneonatalscreening.Whennecessary,thesweattest wasperformedwithpriornoticeandafteractivesearchbya specializedteam,andtheparentswereinformedbymeans ofaquestionnaireappliedatthetestsite.Asthisisa rou-tinetestattheaccreditedservice,aninformedconsentwas notrequired.

Statisticalanalysis

The estimate of the difference between means was per-formedbyStudent’st-testandanalysisofvariance(ANOVA), whereas Pearson’s chi-squared test was used for the differencebetweenfrequencies.Pearson’scorrelation coef-ficientwascalculatedbetweenthequantitativecoulometric method(goldstandard)andtheconductivitytest,withthe kappacoefficientusedtoestimatethecorrelationbetween the methods. A receiver operating characteristic (ROC) curvewasconstructedtoestimatethecutoffpointand sen-sitivityand specificityrates ofthe conductivity test,with the coulometric method employed as the gold standard. Aminimum significancelevel of5%wasconsidered for all testsusingtheStatisticaprogram---Statsoft®_{(StatSoft,Inc.,}

DELL®_,OK,USA).

Results

Table1 QuantitativemeasurementofchlorideandconductivityinpatientswithoutCF.

Mean(SD) 95%CI Median Minimum Maximum

Chloride(n=444) 26.0±32.2 23.0---29.0 12.0 3.0 35.0

Conductivity(n=444) 44.1± 29.5 41.3---46.8 32.0 6.0 48.0

CF,cysticfibrosis;SD,standarddeviation;95%CI,95%confidenceinterval. Chlorideandconductivityvaluesareexpressedinmmol/L.

Table2 QuantitativemeasurementofchlorideandofconductivityinCFpatients.

Mean(SD) 95%CI Median Minimum Maximum

Chloride1(n=90) 91.9± 19.0 87.5---96.3 97.0 60.0 110.0

Conductivity1(n=90) 102.4±18.8 98.5---106.3 108.0 53.0 133.0

Chloride2(n=90) 91.7± 19.0 86.4---96.9 97.5 60.0 115.0

Conductivity2(n=90) 106.4±18.9 102.6---110.2 110.0 57.0 129.0

SD,standarddeviation;95%CI,95%confidenceinterval;CF,cysticfibrosis.

Chlorideandconductivity1and2correspondtothefirstandsecondexaminations,respectively. Chlorideandconductivityvaluesareexpressedinmmol/L.

In the group without CF, 185 (41.6%) were males, 234 (52.7%)females,andfor25cases(5.6%)thisinformationwas notavailable.Asforethnicity,420(94.6%)werewhiteand in24cases(5.4%)ethnicitywasunknown.Themeanbirth weightwas3228.8±2352.8g(95%CI:3002.4---3455.4g).In the group with CF, the distribution between the genders and ethnicity was 46 (51.1%) males, 44 (48.8%) females, 85 (94.5%) whites and 5 (5.5%) non-whites. The mean birthweightwas3091.2±477.9g(95%CI:2000.0---3997.0g). The sweatvolumecollectedfromallchildren wasgreater than 15␮L, allowing both methods to be used. In the

non-CF group, median sweat test results by coulometry and conductivitywere 12mmol/L and 32mmol/L, respec-tively(Table1).In thegroupwithCF,medianvalueswere 97.0mmol/L and 108mmol/L, respectively, showing good correlationbetweenthetests,consideringthatconductivity measuresallionspresentinsweat(Table2).

In the group withCF, 14 (3.15%) showed high conduc-tivity (>50.0mmol/L), but normal chloride measurements (<60.0mmolCl−_{/L), which were considered false positive} results.The resultsshowedexcellent correlationbetween themethodsandpropertiesobtainedwhencomparingthem, consideringthequantitativemeasurementofchlorideasthe goldstandard(Table3).

AllCFpatientsshowedconductivityvalues>80mmol/L, whichgivesthetestasensitivityof100%(95%CI:94.8---100), specificity of 96.2% (95% CI: 93.8---97.8), positive predic-tivevalueof83.3%(95%CI:74.4---91.1),negativepredictive valueof100%(95%CI:90.5---109.4),accuracyof96.8%,false positiverateof16.7%,zerofalsenegativerate,positive like-lihoodratioof31.7(18.9---53.1),andzeronegativelikelihood ratio.Thecorrelationcoefficientbetweenthetwomethods was0.97, with kappa factor of 0.89 (p<0.001). The ROC curve (Fig.1) showed 100% overlap underthe curve with theconductivitymethodandthesuggestedcutoffvaluewas 69.0mmol/L.

Discussion

Theassessmentcarriedoutinthisstudyshowsthe conduc-tivitymethodcanbeusedasadiagnostictool,considering

Table 3 Concordance between theconductivity andthe coulometricquantitativetestforchloridemeasurement.

Coulometrictest Total

Positive Negative

Conductivity

Positive 90 14 104

Negative 0 430 430

Total 90 444 534

Kappa coefficient=0.89; (Fisher’s exact test) p<0.001; sen-sitivity=100% (95.9---100); specificity=96.8% (94.8---98.1); PPV (positive predictive value)=86.5%; NPV (negative predictive value)=100%;accuracy=97.4%;falsepositive=13.5%;false neg-ative=0;positivelikelihood ratio=31.7(18.9---53.1);negative likelihoodratio=0.

Conductivity method

0 20 40 60 80 100

100-specificity

100

80

60

40

20

0

Sensitivity

69.9 mmol/L

thatallstatisticaltestsusedtocompareitwiththe coulo-metricquantitativechloridemeasurement,whichisthegold standard,indicatedstrong accuracy.The cutoffvalue sug-gestedbytheROCcurveanalysiswas69mmol/L,withthe conductivityvaluesof60---69mmol/Lconsideredas equivo-calrangevalues(p=0.001).Theresultsofthisstudysuggest thattheanalyticalmethodofconductivitycanbea defini-tiveandreliablediagnosticprocedureforCF.Thecorrelation coefficient(r=0.97)foundinthisstudyshowsgood conver-gencebetweenthesemethods.

Althoughsomestudieshaveshownthattheconductivity correlateswellwiththechlorideconcentrations,7,13,14,17---19 thisfact is notaccepted by the NorthAmerican National Clinical and Laboratory Standards Institute (CLSI) as a method usedfor definitive diagnosis, relegating it tothe statusof‘‘screeningmethod’’,13,14_{anditrecommendsthat} individualswithsweatconductivityof50mmol/Lbe submit-tedtoquantitativechlorideassessment.

Currently,thequantitativeanalysisofchlorideorsodium isthemostwidelyusedandconclusivemethodforthe diag-nosisofCF,asmorethan99%ofindividualshomozygousfor the CF gene have chloride and sodium in sweat >60 and 70mmol/L, respectively.15,20 _{Some studies have suggested} thatthecutoffforneonatesshouldbe30mmol/L,toprevent thepossibilityoffalsenegativeresults.10,15_{Theelectrolytes} insweat maybe transientlyelevated during thefirst 24h oflife.Forthisreasonitissuggestedthatthetestbe per-formed48hafterbirth.15

The presentstudy hasshownthatconductivity is capa-bleofdiscriminatingCFpatientsfromnormalsubjects,with 16.7% falsepositive results (Table 3). As the conductivity resultsfrom thesum of electrolytesin sweat, theresults areexpectedtobehigher.Themedianconductivityincases without CF were, respectively, 32mmol/L and 12mmol/L and,incaseswithCF,108mmol/Land97mmol/Linthefirst test,and110mmol/Land97.5mmol/L inthesecondtest, performedondifferentdays(Tables1and2).The conduc-tivityresults werealways higher andthere werenofalse negativecases.

Theresultssuggestthatthisanalyticalmethod, accord-ingto the criteriaof 100% sensitivity (95% CI:94.8---100), 99.5%specificity(95%CI:93.8---97.8),andconductivityvalue of69.0mmol/L,shouldbeconsideredconfirmatoryforthe disease.No false-negative CF cases werereported in the screened population, which can occur, asthere aresome mutationsinthe CFTRgenethatcausesweatelectrolytes toremainwithinthenormalrange.7

Studies have shown an excellent correlation between theconductivity andthecoulometrictitrationofchloride. Hammond et al. evaluated conductivity and the chloride concentration in sweat in 471 individuals without CF and found mean values of 33.4mmol/L (13---87mmol/L) and 16.4mmol/L (5---60mmol/L) for the twomethods, respec-tively. In the 43 patients with CF, the mean conductivity valuewas113.1mmol/L(90---136mmol/L),andforchloride, 98.8mmol/L(77---117mmol/L).Therewasagoodcorrelation betweenthemethods(r=0.97).13

Lezanaetal.comparedthetwomethodsin3834patients withclinical suspicion,whose age ranged from1.8 to 54 years;the median conductivity found in patients without andwithCFwas36mmol/Land111mmol/L,respectively. Thecorrelation coefficient betweenthetwomethods was

0.60 (p<0.001).The ROC curveshowed high concordance between them,ruling out CFfor conductivity <75mmol/L andconfirmingCFforvalues≥90mmol/L,with99.7% sensi-tivity,100%specificity,andPPVandNPVof100%and99.97%, respectively.The kappacoefficientwas0.99.14 _Theresults obtainedinthisstudy,inapopulationconsistingessentially ofnewborns,corroboratetheresultsoftheseauthors.

Mastella et al., evaluating 287 individuals, found mean values of chloride and conductivity of 16.3mmol/L

(4---60mmol/L) and 39.8mmol/L (19---87mmol/L) in

patients without CF, respectively; in CF patients, val-ues were 95.7mmol/L (32---121mmol/L) and 112mmol/L (45---173mmol/L),respectively.21

Mattaret al., comparing the classical method of Gib-son and Cooke and conductivity in 738 children, found medianchlorinevaluesandsweatconductivityof11mmol/L and 25mmol/L, respectively, in 714 children. In the 24 patients with CF,the mean valuesfound were 87mmol/L and103mmol/L,respectively.Theseauthorsfounda sensi-tivityof83.3%,specificityof99.7%,PPVof90.9%,andNPV of99.4%,respectively,forconductivityvalues>90mmol/L. TheconductivityvalueusedtoruleoutCFwas<75mmol/L. Goodagreementwasobservedbetweenthemethods(kappa coefficient=0.93).18

Riedietal.found astrongcorrelation betweensodium measurementandconductivityinsweat.Ofthe175patients without CF, the mean values of sodium and conductiv-ity were 36.3mmol/L (12---75mmol/L) and 40.9mmol/L

(16---75mmol/L); in the 31 patients with CF, they

were 113.2mmol/L (80---146mmol/L) and 118.5mmol/L (84---155mmol/L).Althoughsodiummeasurementisnot rec-ommendedinthediagnosisofCF,theseauthorsfoundagood correlationbetweenthemethods(r=0.99)(p<0.0001).19

Khoa-Nguyenetal.demonstratedthelinearitybetween the colorimetric titration and the coulometric and con-ductivity methods in the range of 10---120mmol/L. They establishedacorrespondencerateofthecoulometric mea-surementandtheconductivityratioof30,40,and60mmolL to57,66,and84mmol/L,respectively.Theseauthors sup-portthatasweattestforconductivity<50mmol/Lrulesout the CF diagnosis in the absence of clinical signs.17 _{In the} presentstudy,all90CFcasesshowedaconductivityresult >50mmol/L,confirmingtheseauthors’findings.

The apparent advantage of the Macroduct® _{system is}

the capacityto obtain directreadings of theresults with small amounts of sweat.11 _{Two positive tests carried out} ondifferentdatesarerequired toconfirmthediagnosis.15 Inadequatesweatingcandistorttheresultsandthus,a min-imumacceptablesweatvolumeof15␮Lcollectedin30min is recommended.11,13,15 _{LeGrys et al. defined the} follow-ingassufficientandoptimalsweat volume:atleast 75mg usingthe Gibson---Cooke method22 _{or at least 10---15}

␮L by theMacroduct®_,23 _{recommendationsthatwerefollowedin} thisstudy.

Someauthorsevaluatedtherepetitionrateofthesweat testforinappropriatevolume,andrecommendeda percent-agelimitof5%ofretestinpatientsaged>3months.23,24_In thisstudy,itwasnecessarytorepeatthetestin99children (4.3%) due toinsufficient volume,thus within the recom-mendedrate.

quantitative measurements of chloride, especially those usingfilter paperor gauzeto collectsweat, in which the sample quantity can be a limiting factor. The results of thisstudycorroboratethosepreviouslypublishedbyother authors and reinforce theuse of this test as a diagnostic method,asthe operatorhas less influenceonthe quality ofresults.

Thisstudysuggeststhattheconductivitytestisvalidfor thequantitativeanalysisofchlorideinsweattodiagnoseor excludeCFandcanbeconsideredasadiagnosticmethod.

Conflicts

of

interest

Theauthorsdeclarenoconflictsofinterest.

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