The Pharmaceutical Calculation Issue - Part One

Adopting Best Calculation Practices Part One: Manual Calculations

The Pharmaceutical Calculation Issue - Part One


A recent 60-Minutes report entitled, “In Short Supply”1 described drug shortages as an ongoing, daily occurrence in hospital pharmacies. During shortages, pharmacies often obtain an alternate source of drug, which can require recalculated compounding formulations. Here, we discuss how sterile compounding programs can use best calculation practices to protect their formulations from calculation errors.

A bag of compounded Parenteral Nutrition (PN) is the product of “50 or more individual components”2 in a careful balance of amino acids, dextrose, lipids, electrolytes, minerals, and vitamins. A drug shortage can suddenly disturb that balance.

In addition to those disruptions, the multitude of different calculations that are taught in training programs and pharmacy schools can also be a challenge. Having one calculation method for all situations may not be a practical solution, but adding best calculation practices to your standard operating procedures (SOPs) could safeguard countless compounded sterile preparations (CSPs).

If you’re unsure about what equals best calculation practices, USP <1160> “Pharmaceutical Calculations in Pharmacy Practice,” is a good starting point. For additional insights, we’ve developed four simple axioms, based on USP <1160>, to help you confidently design your own best calculation practices SOP:


Axiom One: Disseminate Information in a Logical Format

USP <1160> states that pharmacists and support personnel should select logical methods “to ensure that pharmaceutical calculations are done accurately and correctly.”3 Likewise, pharmacies should select a logical format for presenting the SOP information to ensure that personnel correctly and accurately follow the content.

For example, if PN terminology is already widely used by your personnel, incorporate amino acids, dextrose, lipids and trace elements in example calculations. Pharmaceutical calculations are an abstract topic. Counteract the conceptual with a dose of familiarity.


Axiom Two: Direct the Reader’s Attention to the Details and Arithmetical Concepts

Correct pharmaceutical calculations are accomplished by understanding arithmetical concepts and “paying close attention to the details.”4 Units of measure (UOM) are a life-or-death detail that must be highlighted in your SOP. Consider how the scales can vary in weight-based calculations.

An infant may be weighed in kilograms (kg), but that same infant could require PN ingredients that are measured in UOMs that are smaller by many orders of magnitude. Some ingredients will be in milligrams (mg), each the equivalent of 1/1000th gram (g). Others will be measured in micrograms (mcg), or 1/1,000,000th of a gram. Mixing up any of these UOMs could result in a tragic error. 


Axiom Three: Develop a Standard, Independent Double-Check Process

Double-checking calculations is strongly recommended by USP <1160>,5 and the Institute of Safe Medication Practices (ISMP) advocates for the use of independent double-checks.6

In an independent double-check, “two people separately check the targeted components of the work process without knowing the results of their colleague.”6 Perhaps the most desired outcome following the implementation of best calculation practices is the adoption of independent, double-checks for manual calculations.


Axiom Four: Demonstrate Pharmaceutical Calculations Applied in Actual Scenarios

Lastly, to make your SOP resonate with personnel, use real-life scenarios. Calculating for a newborn is a real-life scenario that is laden with risk, and drug shortages are routine in hospital pharmacies.

Consider a scenario in which a formulation had been designed to provide a premature infant with 400 micrograms (mcg) of zinc per kilogram (kg) using injectable zinc with a concentration of 10 milligrams (mg) per 10 milliliters (mL). Suppose the pharmacy can suddenly only obtain the zinc in a 25 milligram (mg) per 5 milliliter (mL) concentration.

Figure 1 – Solving a weight-based problem using Dimensional Units Analysis

Figure 1 demonstrates a weight-based calculation used by the pharmacist to adjust for the new source of zinc. This method pays close attention to the UOMs. An independent double-check confirms the 0.096 milliliter (mL) solution, a drug volume that easily fits on the tip of one’s finger, which is a lasting lesson to leave in a best calculation practices SOP. 

Stay Ahead of the Curve

Enhancing operational, quality and standardization initiatives in the hospital pharmacy is within your reach. Through CAPS Consulting, our team of pharmacy experts can help bring your sterile compounding program to the next level.

Elevate Your Sterile Compounding Program

The first step is simple. Connect with a sterile compounding consultant and find out how our programs can be customized to meet your needs.


2  United States Pharmacopeia, General Chapter <797> - Pharmaceutical Compounding― Sterile Preparations; 2008.
3  United States Pharmacopeia, General Chapter <1160> - Pharmaceutical Calculations in Pharmacy Practice.
4  Ibid.
5  Ibid.
6  Published Review of Independent Double Checks Shouldn’t Dissuade Providers from Using Them Judiciously; Institute for Safe Medication Practices; September 25, 2019.