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Ironing out a problem: Measuring pediatric iron products safely

From the May 5, 1999 issue

PROBLEM:: In previous issues, ISMP has advised prescribers to avoid ordering medications by volume, such as milliliters, teaspoons or amps. Similarly, medication doses are sometimes dangerously ordered by the "dropperful." Such an order, "ferrous sulfate 1 dropperful 3 times daily," was recently written for a pediatric patient who weighed 8 kg. There are several problems with this order. First, there is too much room for misinterpretation of what might constitute a dropperful. One individual might consider it to be a dropper filled to the upper calibration mark, which was the intention in this case. Others might consider it to be, literally, a full dropper, and yet others may consider it acceptable to use a household dropper or a dropper supplied for another product. Likewise, another problem is variation in the measurement scales on droppers supplied with different products. One common ferrous sulfate product, FER-IN-SOL by Mead Johnson, contains 15 mg of elemental iron per 0.6 mL. It comes with a dropper that has two 0.3 mL gradations (0.3 mL and 0.6 mL). However, droppers included with some generic equivalent products have a different scale with significant room for overfill. In fact, the total volume of one generic product's dropper, when literally "filled," is 1.8 mL.

Proper dosing of iron in children is critical if iron toxicity is to be avoided. For example, to treat iron deficiency, the usual dose for children 6 months to 2 years, is up to 6 mg/kg/day in 3 to 4 divided doses. In this case, the appropriate dose for the 8 kg child would have been about 48 mg of elemental iron daily, an amount approximated by measuring 0.6 mL (15 mg of elemental iron) on the Fer-In-Sol dropper, and giving it three times daily. However, in this case, the hospital stocked ferrous sulfate drops by the generic manufacturer. With 15 mg of elemental iron per 0.6 mL in the product, if each "dropperful" is measured to the top of the 1 mL scale and given tid, the child would receive a 75 mg total daily dose or about a 50% overdose. Filled to capacity (1.8 mL), a dropperful given tid would deliver a total daily dose of 135 mg, which is close to 20 mg/kg, the amount considered minimally toxic for a child of this weight.

SAFE PRACTICE RECOMMENDATION:The pharmacist who screened the order was confused by the dosing instructions and acted in proper fashion by calling the prescriber for clarification. The ordered dose of elemental iron was changed from "one dropperful" to "15 mg (0.6 mL) three times a day." To avoid confusion, the proper way to prescribe the dose of any medication is by weight, such as mg. However, many liquid products have varying dose measurement devices (droppers, dosing cups, etc.) which may be calibrated only by volume or may indicate a volume that correlates with another product's dose in milligrams. Therefore, it is critical that dosing directions on prescriptions and MARs be expressed as both mg weight and volume (as was done here after the order was clarified.). Also, only the measuring device included in the original packaging of a liquid medication should be used. The Fer-In-Sol package is safer than generics because it has instructions on proper dropper use, including correct filling and expelling of liquid, which is lacking with the generic product. Manufacturers of iron products should match delivery systems as closely as possible.

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