RESPONSE OF KAOLIN ACT TO HEPARIN - EVALUATION WITH AN AUTOMATED-ASSAY AND HIGHER HEPARIN DOSES

Background. Because previous reports suggest that the linear relations hip between celite activated clotting time (ACT) values and heparin so dium is disrupted if values exceed 500 to 600 seconds, this study was designed to evaluate the relationship of kaolin activated clotting tim e (ACT) values to high in vitro heparin concentrations. In addition, t he relationship of kaolin ACT to heparin concentration as determined m anually was compared with that obtained with an automated heparin dose response assay. Methods. Blood specimens were obtained prior to and a fter heparin administration from 41 cardiac surgical patients requirin g cardiopulmonary bypass in this institutional human studies committee -approved study. Five ACT instruments were used to evaluate the respon se of kaolin ACT to manually added heparin at two anticoagulation leve ls: low range (ACT values of less than 500 seconds) and high range (AC T values of 500 seconds or greater). Specimens were also used to measu re kaolin ACT values at three heparin concentrations with an automated heparin dose response assay (HDR) using a Hepcon instrument. Results. A greater response of kaolin ACT to heparin was seen with high-range ACT values than low-range ACT values as illustrated by greater (p = 0. 002) mean slope values (low range, 99 +/- 30 s . U-1 . mL(-1); high ra nge, 128 +/- 50 s . U-1 . mL(-1)). Good correlations were obtained bet ween heparin concentration and either low- or high-range ACT values as demonstrated by mean correlation coefficients (low range, 0.992; high range, 0.982). The response of low-range kaolin ACT values to heparin was greater than that obtained with the automated heparin dose respon se assay as illustrated by greater (p = 0.005) mean slope values (low range, 99 +/- 30 s . U-1 . mL(-1); HDR 82 +/- 21 s . U-1 . mL(-1)). Go od correlations were observed for the relationship between heparin and ACT values obtained with the HDR assay (r = 0.998). Conclusions. A va riable response of kaolin ACT to heparin among patients was demonstrat ed in our study, especially when ACT values exceeded 500 seconds. We f ound that the response of kaolin ACT to higher heparin concentrations was acceptable for clinical monitoring based on good correlations obta ined in individual patients. The HDR assay generally overestimates a p atient's heparin requirements; most likely, this is due to a lower res ponse of kaolin ACT to heparin concentration that is reflected by this assay. Because an exceptional correlation can be obtained between kao lin ACT values and heparin concentration using the assay, this automat ed assay can identify heparin-resistant patients who may need further treatment.