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Dose confusion with phosphorus containing products



From the December 1, 1999 issue

PROBLEM:Confusion with dosing phosphorus supplements has led to numerous errors. Variations in the way prescribers express doses of potassium and sodium phosphate has led to error prone conversions between mg, mEq, and mM for the phosphorous component of products as well as inattention to the amount of potassium or sodium delivered with each dose. Also, oral and parenteral product labels can be confusing. Some list up to five different measurement units (mg, mM, mEq, mOsmol, and mL) when expressing the container's volume and strength of various salts and other ingredients. Recently, label confusion and access to the pharmacy after hours led to an error in which an IV solution containing 132 mEq/L of potassium and 90 mM/L of phosphate was administered to a patient. During the night, an order was written for 30 mEq of potassium phosphate in 1,000 mL IV to run over 6 hours. A nursing supervisor entered the pharmacy and attempted to determine the contents of 15 mL vials of potassium phosphate injection. On the side of the vial, the label stated: "Each mL contains: monobasic potassium phosphate (anhydrous) 224 mg; dibasic potassium phosphate (anhydrous) 236 mg; Water for injection q.s. 7.4 mOsmol/mL. Each mL provides: potassium 170 mg (4.4 mEq); phosphate (H2PO4- + HPO4=) 285 mg (equivalent to 3 mM phosphorus)." The front of the label prominently listed "Potassium Phosphate," followed by "Phosphorus 3 mM/mL" and "Potassium 4.4 mEq/mL" in much smaller print. Below this, "15 mL" was listed in large bold print. The label information overwhelmed the supervisor, and she misread "15 mL" as 15 mM. Believing that 15 mM and 15 mEq were equivalent, she dispensed two vials. The patient's nurse, equally confused, added both vials to the IV fluid and started the infusion. The pharmacy reopened about an hour later and the error was immediately detected. The supervisor documented removal of 2 vials of potassium phosphate 15 mM, but she left a sample of the 15 mL vial on the counter for inspection. The patient received 150 mL of the solution (about 20 mEq of potassium and 14 mM of phosphorus) and serious harm was avoided.

SAFE PRACTICE RECOMMENDATION:Two forms of phosphate (monovalent -H2PO4-1 and divalent - HPO4-2) are present in the normal physiologic range of blood pH. Thus, phosphorus supplements are a combination of monobasic and dibasic salts, the ratio of which is dependent on pH. Because of the polyvalent nature of phosphate anions, mEq should not be used to order or express a given amount of phosphorus. Milliequivalents are calculated by dividing the atomic weight in mg by the valence. Since phosphate anions are polyvalent and the ratio of monovalent to divalent forms is dependent on pH, mEq cannot be derived reliably. To avoid confusion, prescribers should order phosphorus supplements in mM of phosphorus (a measurement immune to pH changes) and mEq of potassium or sodium in parentheses (to avoid overlooking the total amount of potassium or sodium). Phosphorus supplements should be entered into computers as mM of phosphorus and mEq of potassium or sodium. Alerts should be built into the system to warn of excessive doses and computer generated MARs and labels should list both mM of phosphorous and mEq of potassium or sodium. To guide therapy, a written protocol and standard order form should be established (see www.ismp.org for protocol criteria). Access to the pharmacy after hours should be prohibited. In hospitals without 24-hour pharmacy service, a "night formulary" should be created and stocked in a specific dispensing cabinet. If phosphate supplements must be stocked in the cabinet, auxiliary labels should be applied to clearly state the total contents of phosphorous (in mM) and potassium or sodium (in mEq). On-call pharmacists should be contacted for questions and an independent double check system should be established to verify all calculations and drugs removed from the night cabinet. Each morning, pharmacy staff should immediately reconcile all drugs removed from the night cabinet via comparison against the physicians' orders.

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