Screening Test. Introduction:

Transcription

1 Intrductin: Screening Test This is the secnd epidemilgy practical under the Medicine & Sciety Mdule II (FF2613). The fcus f this practical is t intrduce the students t the cncept f screening test, sensitivity, specificity and predictive values. Prblem Overview: Yu are the State Medical Officer fr AIDS/HIV f Negeri Sembilan and yu are expected t cnduct a sentinel surveillance fr HIV amngst; Pregnant mthers (Antenatal Screening) STD clinic patients Therefre yu are required t chse an apprpriate screening methd fr the sentinel surveillance. Calculate the sensitivity, specificity and predictive values f the fllwing tests. Explain which test is the best ne fr the sentinel surveillance. Learning Outcme: At the end f this 1-sessin, the student will be able t: Define sensitivity, specificity and predictive values. Explain what is a gd screening test. Package Designer: Assc. Prf. Dr. Azmi Mhd Tamil Dept f Cmmunity Health Tel: drtamil@medic.ukm.my Page 1 f 12

2 Trigger: Yu are the State Medical Officer fr AIDS/HIV f Negeri Sembilan and yu are expected t cnduct a sentinel surveillance fr HIV amngst; Pregnant mthers (Antenatal Screening) STD clinic patients 1. Define sentinel surveillance 2. Hw are the specimens cllected fr sentinel surveillance? 3. What is the ethical way f cnducting HIV testing? 4. What are the screening and cnfirmatry tests fr HIV? Page 2 f 12

3 Data Infrmatin Sheet-1 Chsing the Apprpriate Screening Test T select the apprpriate screening test, yu did a literature review and cllated the fllwing tables. Calculate the sensitivity, specificity, PPV and NPV f each test t help yu decide. Ttal Present Absent Psitive TP FP TP + FP Negative FN TN FN + TN Ttal TP + FN FP + TN N TP = True Psitive FP = False Psitive FN = False Negative TN = True Negative Sensitivity = TP/(TP+FN) x 100% HIV Enzyme Immun Assay (EIA) Gld Standard EIA (bld) ttal ,000 Specificity = TN/(TN+FP) x 100% PPV = TP/(TP+FP) x 100% NPV = TN/(TN+FN) x 100% HIV Particle Agglutinatin Test Gld Standard PA ttal ,000 HIV Rapid Test Kit Rapid Test Oral Rapid Test Kit Oral Test Kit Gld Standard ttal Gld Standard ttal Define True Psitive, False Psitive, True Negative & False Negative. 5. Define sensitivity, specificity and predictive values. EIA PA Rapid Oral Sensitivity Specificity PPV NPV Which is the best screening test? Page 5 f 12

4 Data Infrmatin Sheet-2 Effect f Prevalence n Sensitivity & Specificity Based n the earlier analysis, HIV EIA, a test with sensitivity f 100.0% and specificity f 99.9% was selected t be used fr the sentinel surveillance in Negeri Sembilan. Yu decided t include the inmates f Pusat Serenti Tampin and Pusat Serenti Jelebu in the sentinel surveillance. Each study ppulatin cnsisted f 10,000 peple. Calculate the PPV, NPV and prevalence rate f HIV fr each study ppulatin. Antenatal mthers Present Absent Ttal Psitive Negative Ttal STD Patients Present Absent Ttal Psitive Negative IVDU Ttal Present Absent Ttal Psitive Negative Ttal PPV = NPV = PPV = NPV = PPV = NPV = Ppulatin Ppulatin with HIV Ppulatin withut HIV TOTAL Antenatal mthers ,000 STD Patients ,000 IVDUs ,000 Prevalence rate 6. Discuss hw PPV & NPV is affected by prevalence rate f the disease. Since the sensitivity and specificity is the same fr all three study ppulatins, please discuss hw PPV and NPV are affected by the prevalence f the disease in each study ppulatin. PPV and NPV can als be calculated using the fllwing frmulas; PPV = Prevalence x Sensitivity (Prev x Sen) + (1 - Prev)x (1 - Sp) NPV = (1-Prevalence) x Specificity (1-Prev) x Sp + Prev x (1 - Sen) Page 6 f 12

5 Data Infrmatin Sheet-3 Effect f Prevalence n Sensitivity & Specificity Hypthetical Illustratin f Screening Prgramme with Test Kit + - Ppulatin 10,000 + a b a+b Sensitivity % - c d c+d Specificity 99.90% a+c b+d a+b+c+d TP FP FN TN PPV NPV Prevalence a b c d a+c b+d a/a+b d/c+d 0.01% , , % % 0.02% , , % % 0.03% , , % % <- Antenatal 0.05% , , % % 0.09% , , % % <- Bld Dnrs 1.00% , , % % 5.00% , , % % 10.00% , % % 20.00% , % % <- Pusat Serenti 30.00% , % % PPV based n Prevalence, Sensitivity & Specificity sensitivity % specificity % prevalence 99% 95% 90% 80% 99% 95% 90% 80% 20.0% 96.1% 82.6% 69.2% 50.0% 10.0% 91.7% 67.9% 50.0% 30.8% 5.0% 83.9% 50.0% 32.1% 17.4% 1.0% 50.0% 16.1% 8.3% 3.9% 0.1% 9.0% 1.9% 0.9% 0.4% Page 7 f 12

6 References: Osman Ali Kaedah Epidemilgi. Penerbit: Dewan Bahasa Dan Pustaka. UNAIDS/WHO UNAIDS/WHO Plicy Statement n HIV Testing WHO (March 1997) Revised Recmmendatin fr the Selectin and Use f HIV Antibdy Tests. Weekly Epidemilgical Recrd, N WHOSEA Standard Operating Prcedures fr Diagnsis f HIV Infectin. CDC What are the different HIV screening tests available in the U.S.? USFDA Dnr Screening Assays fr Infectius Agents and HIV Diagnstic Assays Jseph Hellweg Narrative and Secrecy: Sentinel Surveillance and Alternative Epidemilgies f HIV/AIDS in Nrthwestern Côte d'ivire. Africa Cnference 2005: African Health and Illness. Trisha Greenhalgh Hw t read a paper: Papers that reprt diagnstic r screening tests. BMJ 1997;315: (30 August) Page 8 f 12