Epidemiologic evaluation of diagnostic tests Mr.sci. Sabina Šerić-Haračić TCDC/TCCT consultant – Aquatic epidemiology

 Where to sample  What to sample  How to properly package/ship samples  How many samples  Where to ship  Who is paying Diagnostic consideration in field

 Is sampled material appropriate for test used  Have samples been shipped properly  Condition of samples upon arrival  Having and knowing to use recommended test/s  Where to send results  Who is paying Diagnostic consideration in lab

 Having relevant and detailed sampling, shipping, testing procedures  Level of compliance to prescribed procedures  Do we have all necessary information  Is number of samples tested enough to safely conclude presence/absence of disease  Are test/s used precise enough to confirm/rule out disease  How much this costs Diagnostic consideration in decision making

DiseasedHealthy Test positive Test negative Test sensitivity and specificity Are test/s used precise enough to confirm/rule out disease

DiseasedHealthy Test positive Test negative Test sensitivity and specificity

Se/Sp and prevalence UNKNOWN!!! KNOWN!!!

 Positive tests results came for  92% of diseased animals  3% healthy animals  What is test Sensitivity and Specificity for  Prevalence = 7,3%  Prevalence =15% Assignment DiseasedHealthy Test positive Test negative

 Prevalence = 0  SP=90%  Out of 100 healthy animals we will have 5 positive results (false positive)  Repeat testing  Use another test  Use better test  Demonstrating disease freedom requires Sp=100% SE/SP and disease freedom

 Fair coin -50% cahnce of heads/tails  Assume unfair coin – 90% of heads 10%of tails  Number of trows – 20  Chance to have all heads from 20 trows if chance head in each trow is 90% Test results on herd level (farm/pond/pooled samples...)

 Result of a test from at least 2 animals or from pooling of 2 samples  Interpretation of the herd status is more important than interpretation of each individual's test results  Herd Sensitivity is the probability that an infected herd has a positive herd test result  Herd Specificity is the probability that an non- infected herd has a negative herd test result Test results on herd level (farm/pond/pooled samples...)

 Factors affecting Herd Sensitivity (HSe) and Herd Specificity (HSp)  Individual Se and Sp  Within herd prevalence of disease  Number of animals tested in the group  Number of reactor animals per group that will designate a positive or negative herd Test results on herd level (farm/pond/pooled samples...)

 For and infected herd (P>0): For a non-infected herd (TP=0):  Test results on herd level (farm/pond/pooled samples...) Probability of obtaining a positive testAP=p(T+)=Se * P + (1-P)*(1-Sp) Probability of obtaining a negative test1-AP Probability of finding zero positive individuals (false negative herd proportion) (1-AP) n Herd Sensitivity (HSe)1-(1-AP) n Probability of obtaining a positive test (replace P=0 in eq 1) AP= (1-Sp) Probability of finding at least one positive animal (false positive herd proportion) 1-Sp n Probability of obtaining a negative test1-AP = p(T-) = 1-(1-Sp)=Sp Herd Specificity (HSp)Sp n

Example/Assignment 1 100% 100 0,

Example/Assignment 100 0,