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.