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It doesn't pay to play the percentages

From the October 16, 2002 issue

PROBLEM: Several incidents have been reported where undiluted epinephrine 1:1,000 (1 mg/mL) was given IV to patients instead of using the 1:10,000 (0.1 mg/mL) concentration. In each case, the more diluted epinephrine (1:10,000) was available for use, but staff inadvertently prescribed or selected the 1:1,000 concentration. One error occurred in an outpatient radiology unit where the nurse on duty rarely administered medications. The patient developed a reaction to contrast media, with visible hives and respiratory distress. The physician prescribed 3 mL of the 1:10,000 concentration IV, but 3 mL of the 1:1,000 concentration was administered in error. Lack of understanding the difference between the two concentrations led to the error, but an additional problem is that the dilutions are hard to differentiate since 1,000 looks like 10,000. In another case, a physician's assistant ordered the incorrect concentration for a patient in an urgent care clinic, and the nurse administered it without recognizing the problem. In fact, a warning to dilute the 1:1,000 concentration before IV administration is not mentioned anywhere on the ampuls of these products. Both patients developed rapid heart rates and increased blood pressures, necessitating an overnight stay in the hospital.

These errors highlight an ongoing problem of drug concentration presentation. Typically, the contents of most injectable medications are given as their mass concentration (mg or mcg per mL). But a few drugs have concentrations expressed as a dilution ratio or percentage (e.g., epinephrine 1:1,000, lidocaine 1%). These expressions are error-prone, especially in light of studies that have shown that knowledge about concentrations expressed as a ratio or percentage is inadequate, even among physicians and emergency medicine residents.1-3

Most alarming, these poorly understood expressions are particularly prevalent with drugs used for resuscitation (e.g., calcium, epinephrine, lidocaine, magnesium sulfate, neostigmine, sodium bicarbonate). Thus, an inappropriate dose or life-threatening delay in treatment is quite possible, especially if these drugs are prescribed in mg (which requires prior knowledge of ratio or percent concentrations and calculations) or mL (which is a problem if multiple concentrations exist). To cite one example, at a neonatal code recently, an epinephrine dose was ordered in mL. Despite a good deal of confusion at first, a pharmacist who attended the code was able to guide staff regarding the proper dose for the patient since both 1:1,000 and 1:10,000 dilutions were available on the code cart. Neonatal nurses and physicians had assumed that only the 1:10,000 dilution was available on the code cart.

SAFE PRACTICE RECOMMENDATION: Based on research findings and everyday practice experiences, do not expect all health care practitioners to be familiar with percent or ratio expressions of concentrations, or to be adept at calculating doses for drugs with concentrations expressed in this manner. Therefore, it would be helpful to create a dose conversion chart reflecting concentrations that are available in your facility and post them on code carts and in other areas where emergency medications may be prepared. Since an independent double check by another clinician may not be feasible in emergency situations, encourage staff to refer to the dose chart before administration of these products.

During annual CPR certification for clinical staff, be sure to review the dose chart and mention potential confusion with emergency drugs dosed in ratio or percent concentrations. Also, point out the hazards of ordering injectable medications by volume alone. Store a single concentration wherever possible and affix warning labels as appropriate to minimize confusion between the two concentrations of epinephrine. In some locations, such as a code cart, it might be feasible to store epinephrine in 1:10,000 pre-filled 10 mL syringes as the only choice. In units where multiple concentrations may be needed (such as the ER), consider applying auxiliary warning labels to the 1:1,000 ampuls to alert staff to the concentration in mg and to dilute it before IV use.

Since many of the emergency medications with concentrations expressed in ratios or percentages date back to before the 1938 Food Drug and Cosmetic Act, they do not fall under current FDA labeling standards. However, since the issue is so serious, we have asked FDA to address the problem. While a ratio or percent expression of concentration may be less dangerous for topical use products or in local anesthetics, they do not serve us well when dosing is systemic.

References: (1). Rolfe S, Harper NJ. Ability of hospital doctors to calculate drug doses. BMJ 1995; 310:1173-4. (2). Jones SJ, Cohen AM. Confusing drug concentrations. Anaesthesia 2001;56:195-6. (3). Nelson LS, Gordon PE, Simmons MD, et al. The benefit of houseofficer education on proper medication dose calculation and ordering. Academic Emergency Medicine 2000; 1311-16.

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