Weight for weight based drugs

Here’s a conversation I’ve had with an RN preparing to give a weight based medication via a peripheral IV:

 I don’t weigh the patient each time they come in for an infusion. I only weigh them at the beginning of their treatment to get a baseline weight. Why should I weigh them again, they can tell me if their weight has changed.

Weighing patients is one of the routine procedures done in any patient care setting, whether in a hospital, outpatient clinic or in a doctor’s office. It’s one of the things I dread when I go to my primary physician, I don’t like to know how much weight I gained!!!  My weight may not be significant to what I came to see the doctor for, but I know I am always weighed each visit. And there is no way around it, you are weighed, then a thermometer is stuck in your mouth and a BP cuff around your arm!! It is that routine!!

This statement from an RN was surprising to me. Why not weight the patient and get a current weight? Sure, their weight may not have changed from the last visit two weeks ago, but how do we know it didn’t change if patients are not weighed? There is a scale always available to use, and if patients are able to ambulate, it doesn’t take a lot of time to weigh them. You are about to calculate a weight based drug for infusion, why are you not weighing the patient to get a current weight?  Is it good practice to rely on the patient to report that their weight has changed? Relying on patients reporting their weight is acceptable only if there is no way a patient can be weighed.

Bottom line: this vital piece of patient-specific information, the patient weight, is especially important because it is used to calculate the appropriate dose of a medication (e.g., mg/kg, mcg/kg, mg/m2). A prescribed medication dose can differ significantly from the appropriate dose as a result of missing or inaccurate patient weights. There is little information in the literature that specifically mentions medication errors that result from missing or inaccurate patient weights. But one from 2001, a prospective, cohort study of 1,120 patients in two academic institutions revealed that 3.7% of the institutions’ medication errors were due to missing or wrong weights. If the patient can’t be weighed, an estimated weight might be used instead but still leaves room for inaccuracies.

As my conversation continued with this RN, the reason why infusion patients are not weighed became obvious – the scale was on the opposite end of where patients are infused and too out of the way for the patients (who are ambulatory) and the nurse!! Ah, there we go!!

So dear colleagues, is this a good enough reason not to weigh patients? Would love to hear your thoughts…



  1. Kaushal R, Bates DW, Landrigan C, et al. Medication errors and adverse drug events in pediatric inpatients. JAMA 2001 Apr 25;285(16):2114-20.