Critical values are vital to safe clinical and laboratory practice. To address the lack of information on critical values in coagulation, pattern-of-practice surveys were distributed to members of the North American Specialized Coagulation Laboratory Association. More than 70% of respondents had critical values for commonly performed tests. Median values were as follows: prothrombin time, more than 37 seconds; international normalized ratio, more than 5; activated partial thromboplastin time, more than 100 seconds; and fibrinogen level, less than 100 mg/dL. Critical value reporting generated a significant workload, with up to 15% of these tests yielding critical results. The median time to report critical values was 7 minutes for inpatients. Despite the lack of guidelines surrounding critical values in coagulation, this survey confirms that laboratories have reasonable and uniform practices. It also provides critical value medians and ranges for a wide range of tests. Laboratories without critical values or in the process of reviewing their values may find this survey of their peers useful.