Skip to main content Back to Top

8/28/2001

Anticoagulation Clinics Know Their INRs

Kate Traynor

Pharmacists who run anticoagulation clinics say their programs offer advantages over less-specialized settings for patients receiving warfarin therapy.

"I honestly believe that the beauty of an anticoag clinic is that you have pretty much the same folks taking care of these patients day after day," said Claudia D. Ogburn, pharmacy director at the Fort Sanders Sevier Medical Center in Sevierville, Tenn. "You have great communication between the patient and the health professionals.... It’s probably the ideal situation" for monitoring therapy.

No anticoagulation clinic exists at St. Agnes Medical Center in Philadelphia, where laboratory errors apparently led to the warfarin-related deaths of two elderly men in June. The men were among 932 patients whose prothrombin time was measured between June 4 and July 25, when the mistakes occurred. These errors resulted in the reporting of inaccurately low International Normalized Ratios (INRs) and subsequent warfarin-dosage increases for some patients.

The hospital has acknowledged that the laboratory errors caused the two patient deaths but emphasized that not all 932 patients whose blood was tested in June or July were receiving warfarin therapy.

"A small percentage of the people that had the blood tests were on Coumadin," said Teresa L. Heavens, vice president for advancement at St. Agnes, referring to the warfarin sodium product by Du Pont Pharmaceuticals Co. Heavens said most prothrombin-time determinations made at St. Agnes are for routine preoperative screening of the hospital’s elderly patients.

Compared with a setting in which each physician monitors the INRs of only his or her patients, an anticoagulation clinic run by a pharmacist might more readily detect a string of unusual readings among a group of warfarin users.

"When you run a clinic, I think the advantage is that we have a large number of values and we can see trends," said Michelle B. McElhannon, Pharm.D., director of the anticoagulation management service at the Athens Regional Medical Center in Georgia.

"We do get a large number of just INRs in a day," McElhannon said. She speculated that "even a clinic who is using a lab for their lab data...would see a trend soon."

At St. Agnes, some patients’ INR values were miscalculated through the use of an incorrect International Sensitivity Index (ISI) rating—a number that can change when a new lot of a thromboplastin reagent is used to test patients’ blood-clotting tendency.

In contrast, both Athens Regional and Fort Sanders rely on point-of-care devices that analyze blood placed in a specially treated cuvette or card rather than collected into a glass tube. McElhannon’s device automatically calculates INRs by using an ISI preset by the device’s manufacturer.

"I can’t order a different sensitivity cuvette," McElhannon said. "So there’s one source where the error that happened at St. Agnes could not occur here."

Both McElhannon and Ogburn said their clinics’ devices, which use a sample of whole blood obtained from a finger stick, are periodically calibrated to ensure they give accurate results. Even so, questionable INRs arise on occasion.

"We have had instances in the past when, all of the sudden, we’ve had too many patients who are off target," Ogburn said. "And then we’ve stopped to say ‘Wait—let’s do arm sticks.’ And we’ve sent our blood to a different lab just to verify results."

McElhannon concurred on the need to question some results. "Even with a finger stick you can have sources of potential error. If we suspect a test result may be inaccurate, the test can easily be repeated immediately or even correlated with the laboratory."

McElhannon said the long-standing relationship with clinic patients helps the staff spot irregularities. "When you know your patients, you know the questions to ask and you know when something is off," she said.