T hromboembolic (TE) prophylaxis using lowdose unfractionated heparin (LD-UFH), ie, 5000 IU given subcutaneously twice or 3 times daily is effective, has low bleeding risk, and is generally not expected to prolong partial thromboplastin times (PTTs). Laboratory monitoring or dose adjustments based on weight or renal function are not considered necessary. 1 An index case at our institution prompted us to review this conventional view and investigate the frequency and circumstances when

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... t PTT prolongations occur in hospitalized patients receiving LD-UFH for TE prophylaxis. Report of a Case. A 73-year-old woman with diabetes, hypertension, hyperlipidemia, and chronic renal failure was treated on the inpatient medicine service for hypertensive emergency associated with chest pain and shortness of breath. She was placed on LD-UFH therapy, 3 times daily, for TE prophylaxis on hospital day 1 and improved steadily. On hospital day 11 she was to be discharged, when she was noted to be severely hypotensive. Evaluation revealed a large retroperitoneal hemorrhage with active bleeding from a lumbar artery branch, which was stopped by embolization treatment. The patient was transfused 14 red blood cell units during this episode. She was eventually discharged in stable condition to a skilled nursing facility on hospital day 69 following a complicated course. The first PTT value, 10 days after the start of LD-UFH therapy, when the retroperitoneal hemorrhage was recognized, was 105.4 seconds (reference range Ͻ37.6 seconds [STA-Compact, STA PTT automate reagent; Diagnostica Stago Inc, Parsippany, New Jersey]). Therapy with UFH was stopped, and PTTs were shortened to 35.5 seconds within 19 hours. The cause of the retroperitoneal bleeding and whether LD-UFH played a direct role could not be determined. However, the high PTT result, which was at the upper end of the therapeutic range for UFH at our institution (75-108 seconds, corresponding to anti-Xa levels of 0.3-0.7 U/mL), was alarming during this life-threatening bleeding episode, caused confusion about the validity of the result, and raised questions about the most appropriate therapeutic action. Comment. This case prompted us to look for patients receiving LD-UFH with prolonged PTTs. During the 6-month period since the original case, we identified an additional 15 patients at our hospital with peak PTTs 1.5 times above baseline or greater that were temporally associated with LD-UFH therapy (Figure). In 4 of the 16 patients, PTT or anti-Xa testing was performed by outside reference laboratories on split samples, confirming the high PTTs at our