Verma et al. (A-14) examined the use of heparin-PF4 ELISA screening for heparin-

Question

Verma et al. (A-14) examined the use of heparin-PF4 ELISA screening for heparin-induced thrombocytopenia (HIT) in critically ill patients. Using C-serotonin release assay (SRA) as the way of validating HIT, the authors found that in 31 patients tested negative by SRA, 22 also tested negative by heparin-PF4 ELISA.

a) Calculate the specificity of the heparin-PF4 ELISA testing for HIT.

b) Using a "literature derived sensitivity" of 95% and a prior probability of HIT occurence as 3.1%, find the positive predictive value.

c) Using the same information as part (b), find the negative predictive value.

Explanation / Answer

a) P(T'|D') = 22/(9+22) = 22/31 = 0.7097

b) Sensibility = TP / (TP + FN) = P (TP/HIT) = P (TP and HIT) / P (HIT) = 0.95 ; P (HIT) 0.031

then 0.95 = % TP / 0.031 if we consider the number of patients 31 - TP = 31 * 0.95 * 0.031 = 1

PPV = TP / (TP + FP) as Specificity = TN / (TN + FP) = 22 / (22 + 9) then FP = 9 => PPV = 1 / (1 + 9) = 10 %
c) NPV = TN / (TN + FN) we must calculate FN then

0.95 = 1 / (1 + FN) <=> FN = 0

NPV = 1 / (1 + 0) = 100 %

Related Questions

Navigate

• Browse (All)
• Browse V
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.