Abnormal biphasic waveform (BPW) patterns were previously reported when the activated partial thromboplastin time (aPTT) was performed in plasma from patients with disseminated intravascular coagulation (DIC). In this study, the prevalence of the BPW was examined in a cohort of 508 hospitalized patients with elevated fibrinogen degradation products (FDP) levels (>10 microg/mL). The presence of a BPW was automatically flagged by the MDA analyzer when the slope of the precoagulation phase in the waveform exceeded a threshold value of -0.25%T/sec. In our cohort, 76 patients (15%) were diagnosed with overt DIC according to the criteria recently proposed by the International Society of Thrombosis and Haemostasis (ISTH), whereas 96 patients (18.9%) were diagnosed with DIC following the criteria of the Japanese Ministry of Health and Welfare (JMHW). The JMHW and ISTH criteria agreed in 93% of cases (kappa coefficient 0.76). The concordance between both scoring systems was high in patients with infection but low in solid cancer. The BPW appeared in 65 patients (12.8%), with the highest prevalence (23.6%) in patients with infection. The BPW was more prevalent in the subgroup of patients with DIC: 59.2% and 47.9% for DIC diagnosed by ISTH and JMWH scores, respectively. The prevalence of the BPW was particularly high in patients with DIC and infection: 86.4% and 75.0% for DIC diagnosed by ISTH and JMWH scores, respectively. For the total cohort, the presence of the BPW was significantly associated with DIC. Odds ratios were 29.9 and 19.0 for ISTH and JMWH scores, respectively (p<0.0001). The BPW showed a moderate sensitivity (59.2% for the ISTH score; 47.9% for the JMWH score), but a high specificity (95.4% for both scores). Waveform analysis of the aPTT potentially provides a practical tool in risk assessment of critical care patients, in whom development of DIC is known to worsen the prognosis.