College of American Pathologists
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Calling time-out on digoxin results

March 2004
Terri Yablonsky Stat

Amid all the talk of medication error, how labs measure digoxin levels is as important as ever. Thatís why the CAP will offer a Q-Probes on digoxin measurement and patient safety this fall.

"There is already enough variability in digoxin level because of individual patient sensitivity," says Frederick A. Meier, MD, director of regional pathology services, Henry Ford Health System, Detroit, and one of the Q-Probesí coauthors. "If we add lab variability, we are making it harder for the clinician to make a decision. It’s confusing enough as it is, and weíre making it more confusing."

Digoxin toxicity accounts for about 0.4 percent of all hospital admissions, and between one and two percent of all patients taking digoxin are toxic at any one time, he says. The aim of the CAP study is to reduce the number of uninterpretable elevated digoxin levels.

Used in patients with congestive heart failure, atrial fibrillation, and other cardiac conditions, digoxin may be the most commonly used drug with a significant toxic profile, Dr. Meier says. After ingesting digoxin, patients must wait six to eight hours before having blood drawn to ensure that the drug is distributed adequately throughout the body.

But a 1999 study in American Family Physician looked at 3,434 digoxin determinations obtained in 2,009 patients and found that the timing of blood sampling was appropriate in only 62 percent of the 320 instances of elevated serum digoxin concentrations. Overall, nine percent of samples had elevated blood levels of digoxin. Of that nine percent, about one-third was documented as having been drawn six hours or less after the last digoxin dose was administered. Sampling time could not be determined for another third. Therefore, the elevated level of digoxin was not interpretable in two-thirds of cases, Dr. Meier says.

"The lab has become the little boy who cried wolf," he says. Clinicians therefore tend to regard elevated digoxin levels with suspicion, and they often request another blood sample before taking action.

The CAP Q-Probes study will ask laboratory participants to record the patient’s name, physician, time of last dose, time blood was drawn, and level of digoxin, says Peter J. Howanitz, MD, professor of pathology and vice chair and director of laboratories, SUNY Downstate Medical Center, Brooklyn, and a coauthor of the upcoming Q-Probes. "We will also track how many times patients who enter the study will not have their blood drawn because of hospital policies," he adds.

The intent of the study is to reduce the number of medical errors. "We think digoxin is a candidate for that, because it is a commonly used drug and has a narrow therapeutic range with a significant drug toxicity and mortality rate," Dr. Meier says.

Dr. Howanitz predicts that a number of patients in the CAP study will be shown to have toxic levels only because the specimen was obtained immediately after the drug was ingested. "We teach all physicians that there is an appropriate time for therapeutic drug monitoringóright before the next dose," he says. Levels should also be tested when dosing or patient status has changed.

The CAP study, in which several hundred labs are expected to enroll, may serve as a model for the investigation of other therapeutic drugs, Dr. Howanitz says. Laboratories monitor up to 20 drugs with consequences similar to those of digoxin when samples are drawn too soon.

Orders for study materials and forms should be in by early September so laboratories have sufficient time to collect the data, says Linda Palicki, MT(ASCP), CAP technical analyst in proficiency testing.

After the CAP analyzes the participantsí Q-Probes data, it will make recommendations based on the literature and participantsí suggestions. One recommendation may be for patients to take digoxin at night. "That makes sense,î Dr. Howanitz says. ìEvery time we do a Q-Probes study, someone in the laboratory figures out something that hasnít made it to the literature yet."

Terri Yablonsky Stat is a writer in Evanston, Ill. For more information about the "Patient Safety with Digoxin Measurements" Q-Probes, or to enroll, call CAP customer service at 800-323-4040 or 847-832-7000 option 1#.