Criteria for ScreeningCriteria for ScreeningP. Sri SowmyaP. Sri SowmyaRoll No: 93Roll No: 93

Guided by Sipra Guided by Sipra Madam Madam

ConsiderationsConsiderations

Criteria for screening based on Criteria for screening based on 2 considerations: 2 considerations:

A.A. Disease to be screened Disease to be screened B.B. Test to be appliedTest to be applied

A. Disease to be A. Disease to be ScreenedScreened

Important Health Problem ( High Important Health Problem ( High Prevalence )Prevalence )

Should have a Early Asymptomatic stageShould have a Early Asymptomatic stage

Natural History should be understoodNatural History should be understood

Test to detect the diseaseTest to detect the disease

Facilities to confirm diagnosisFacilities to confirm diagnosis

Effective TreatmentEffective Treatment

Policy on whom to treat as patientsPolicy on whom to treat as patients

Good evidence - reduction of Morbidity & Good evidence - reduction of Morbidity & Mortality Mortality

Expected benefits should exceed risk & costExpected benefits should exceed risk & cost

B. Test to be AppliedB. Test to be AppliedMust satisfy : Must satisfy :

1.1. AcceptabilityAcceptability2.2. Repeatability Repeatability 3.3. Validity Validity

Others : Yield , Others : Yield , Simplicity , Safety, Simplicity , Safety, Rapidity, Ease of Rapidity, Ease of Administration & CostAdministration & Cost

1. Acceptability1. AcceptabilityAcceptability to aimed Acceptability to aimed PeoplePeopleGenerally tests are Generally tests are painful, discomforting painful, discomforting & embarrassing & embarrassing Ex: Rectal or Vaginal Ex: Rectal or Vaginal ExaminationExamination

2. Repeatability 2. Repeatability Also called Also called Reliability, Precision Reliability, Precision or Reproducibility or Reproducibility Consistent Results - Consistent Results - Same individual Same individual & conditions & conditions3 Factors : 3 Factors :

Observer variation Observer variation

A.A. Intra ObserverIntra Observer

B.B. Inter ObserverInter Observer

Biological VariationBiological Variation

Erros Relating to technical methodsErros Relating to technical methods

a. Intra-Observera. Intra-Observer

Also called Within Observer VariationAlso called Within Observer Variation

Same ObserverSame Observer

2 measurements 2 measurements

Same PersonSame Person

Same Time Same Time

Each TimeEach Time

Different Results Different Results

Minimised by - taking average of all Minimised by - taking average of all measurements measurements

++

++

==

==

AA BB

AA BB

XX

YY

b. Inter - Observerb. Inter - ObserverAlso called as “Between - observer variation”Also called as “Between - observer variation”

Different observersDifferent observers

Same subjectSame subject

Ex: Examination of blood Ex: Examination of blood smear for malarial smear for malarial parasite by 2 parasite by 2 observers observers

++

++

==

==

AA CC XX

YYCCBB

Seen in X - rays, ECG, Tracing ,readings of Seen in X - rays, ECG, Tracing ,readings of blood pressure and pathological studiesblood pressure and pathological studies

Error is minimised by Error is minimised by

A.A. Standardisation of procedureStandardisation of procedure

B.B. Intensive trainingIntensive training

C.C. 2 or more observers2 or more observers

Errors never eliminated completely Errors never eliminated completely

Biological VariationBiological VariationAlso called subject variation Also called subject variation

Associated with blood pressure , Associated with blood pressure , blood sugar, serum cholesterol etcblood sugar, serum cholesterol etc

Changes in the parameters Changes in the parameters observed : Ex: Cervical smears , observed : Ex: Cervical smears , Myocardial infarction, BPMyocardial infarction, BP

Variation in the way patient Variation in the way patient perceives the symptomsperceives the symptoms

Regression to MeanRegression to MeanTendency of values at the Tendency of values at the extreme of a distribution to extreme of a distribution to regress towards the mean or regress towards the mean or average on repeated average on repeated measurementsmeasurements

Ex: Pain of Rheumatoid Ex: Pain of Rheumatoid arthritis , Stool frequency in arthritis , Stool frequency in ulcerative colitis, blood ulcerative colitis, blood pressure in Hypertension, pressure in Hypertension, Blood sugar in Diabetics Blood sugar in Diabetics

Checking of ErrorsChecking of ErrorsObserver variation - repeated measurements Observer variation - repeated measurements at at Same timeSame time

Biological variation - repeated measurementsBiological variation - repeated measurements over timeover time

Errors related to technical methods: defective Errors related to technical methods: defective instruments, error in calibration, faulty instruments, error in calibration, faulty reagent,unreliable testreagent,unreliable test

3 .Validity3 .ValidityAlso called as AccuracyAlso called as Accuracy

Extent to which the test accurately Extent to which the test accurately measures measures

2 components : Sensitivity, Specificity 2 components : Sensitivity, Specificity

Sensitivity, Specificity, Predictive Sensitivity, Specificity, Predictive Accuracy - Inherent properties of Accuracy - Inherent properties of Screening test Screening test