Microsoft Word - Reference Range and What They Mean

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ReferenceRangesandWhatTheyMean

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The"Normal"orReferenceRange

Inreferencetoalabtest,whatdoes"Outofthenormalrange"actuallymean?Isitcauseforconcern?

Thebriefansweristhataresultoutofthenormalrangesignalstheneedforfurtherinvestigation–oratleast

furtherexplanation.Interpreting

anyclinicallaboratorytestinvolvescomparingthepatient’sresultstothetest’s

"referencerange"alsocommonlycalledthe"normalrange"or“referenceinterval.”

Whatisareferencerange?

Sometestsprovideasimpleyesorno(positiveornegative,reactiveornon‐reactive)answer.Wastheurineor

bloodpregnancytest

positiveforpregnancy(indicatingthepresenceofahormonecalledHCG)ornegative

(absenceofHCG)?Didthetestfindantibodiestoavirusorbacteriumthatindicatesaninfection?Somelabsreport

thesetestsasreactive(positive)ornon‐reactive(negative).

Morecommonly,themeaningoftestresultsdepends

ontheircontext.Atypicallabreportwillprovideyourresults

followedbyareferencerange.Forexample,yourresultsforathyroid‐stimulatinghormone(TSH)testmightlook

somethinglike:2.0mIU/L(refrange0.5–5.0mIU/L).Thetestresultsindicatethatyourresultfallswithinthe

normalrange.

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Howwasthatreferencerangeestablished?Theshortansweris:bytestingalargenumberofhealthypeopleand

observingwhatis"normal"forthem.Thefirststepindeterminingagivenreferencerangeistodefinethe

populationtowhichthereferencerangewillapply–forexample,healthy

femalesaged20‐30yearsold.Alarge

numberofindividualsfromthiscategorywouldbetestedforaspecificlaboratorytest.Theresultswouldyielda

normal"distributionandareferencerange(plusorminus2standarddeviationsoftheaverage)ofnormally

distributedvalueswouldbeestablished.

Theterm

"referencerange"ispreferredover"normalrange"becausethereferencepopulationcanbeclearly

defined.Ratherthanimplyingthatthetestresultsarebeingcomparedwithsomevaguedefinitionof"normal,"the

referencerangemeanstheresultsarebeingconsideredinthemostrelevantcontext.Whenyouexaminetest

results

fromdifferentpopulations,youquicklydiscoverthatwhatis"normal"foronegroupisnotnecessarily

normalforanothergroup.Forexample,pregnancychangesmanyaspectsofthebody’schemistry,sopregnant

womenhavetheirown"normalrange"formanylabteststhatvarygreatlyfromnon‐pregnantwomenofthe

same

age.

Finally,referencerangesarespecifictothelaboratorythatproducesthetestresults.Formanytestspecimens

(“analytes”),differentlaboratoriesusedifferentkindsofequipmentanddifferentmethodsoftesting.Thismeans

thateachlaboratorymustestablishitsownreferencerangesusingdatafromitsownequipmentand

methods.The

laboratorymustsupplyyourtestresultwithanaccompanyingreferencerangeonthelaboratoryreport.

Consequently,thereisnosuchthingasastandardreferencerange.

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Ofcourse,eachtestdoeshaveatheoreticalreferencerange,whichcanbefoundinmanybooksandonline

sources,butitmayhavelittlediagnosticmeaningforyou.Youandyourdoctorshouldapplythereferencerange

suppliedbythelaboratoryperformingthetest.

EffectsofAgeandSex

For

manytests,thereisnosinglereferencerangethatappliestoeveryonebecausethetestsperformedmaybe

affectedbytheageandsexofthepatient,aswellasmanyotherconsiderations.Therefore,aspartofthe

specimencollectionandsubmissionprocessandthesubsequentreportingofresults,age,

sexandotherfactors

comeintoplayinestablishingthe"normalorreferencerange"andwillbelistedontheresultssheetalongwiththe

resultstoallowproperinterpretation.

Laboratorieswillgenerallyreportthefindingsbasedonageandsex,andleaveittothephysiciantointerpretthe

results

basedonfactorssuchasdiet,yourlevelofactivity,ormedicationsyouaretaking.Ifyouknowofany

specialcircumstancesthatcouldaffectatest,mentionthemtoyourdoctor;don’tassumeyourdoctorhasthought

ofeverypossiblecircumstance.Forexample:Didyourunamarathonyesterday?

Haveyoustartedanew

supplement,medicationofextremediet?

OtherFactorsAffectingTestResults

Laboratorieswillgenerallyreportyourtestresultsaccompaniedbyareferencerangekeyedtoyourageandsex.

Yourphysicianwillstillneedtointerprettheresultsbasedonpersonalknowledgeofyourparticulars,includingany

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medicationsorherbalremediesyoumaybetaking.Aplethoraofadditionalfactorscanaffectyourtestresults:

yourintakeofcaffeine,tobacco,alcoholorvitaminC;yourdiet(vegetarianvs.carnivorous);stressoranxiety;ora

pregnancy.Suchfactorsasoccupation,altitude,anddistancefromtheoceanhavebeen

knowntoaffectresults.

Regularexercisecanalsoaffectvalues.

Alltheseconsiderationsunderscoretheimportanceoftakingbloodorurinesamplesinastandardizedfashionfor

performingandinterpretinglaboratorytests(andhometestsaswell).It’simportanttocomplywithyourdoctor’s

instructionsinpreparingforthetest,such

ascominginfirstthinginthemorningbeforeyoueatanythingtoget

yourblooddrawn.Thatcompliancemakesyoursampleascloseaspossibletoothers;itkeepsyouwithinthe

parametersofyourreferencegroup.

Whatdoesitmeanifmytestresultisoutofthe

referencerange?

Bydefinition,1outof20(or5%)resultswillfalloutsidetheestablishedreferencerangewithspecimenstaken

fromarandomsampleofhealthyindividuals.Generally,ifthetestvalueisonlyslightlyhigherorlowerthanthe

referencerange,thereisusuallynoreasonforalarm.

yourdoctorsuspectsthattheresultmayindicateaproblem,thefirstactionisusuallytorerunthetest.As

accurateasmodernlabtestsareunderoptimaltestingcircumstances,manyfactorsthatyouoryourdoctor

cannotcontrol(heat,humidity,transporttime,etc.)canaffectanygiventestresults.

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Insummary

Remember,an"abnormal"or"outsidethereferencerange"testresultisanotalwaysasignofaproblem.Atest

resultoutsidethereferencerangemayormaynotbeareasonforconcern;youcouldevenbe"betterthan

expected"andstillfalloutsidetheexpectedstatistical

range.Althoughyoucanhavean"abnormal"or"outsidethe

referencerange"valueandhavenothingwrong;usuallyyourdoctorshouldtrytodeterminethecauseifthe

reasonisnotreadilyapparent.

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