Bayesian approach to estimating the probability of HIT. The 4T score in conjunction with an immunoassay can be used to estimate the posttest probability of HIT. The pretest odds of HIT, based on the 4T score, is derived from Cuker et al.³  For likelihood ratios for various immunoassays, see Table 2 in the article by Nagler et al that begins on page 546.¹  The likelihood ratios for the polyspecific ELISA (low threshold) are shown as an example. These values can be used to calculate the posttest odds and posttest probability of HIT using the equations below. ^aPosttest odds = pretest odds × likelihood ratio; ^bPosttest probability = posttest odds/(posttest odds + 1). Professional illustration by Patrick Lane, ScEYEnce Studios.