Q&A: Are there rounding rules for weight based dosing?

Q:  Unless I was absent during class on weight based dosing, I was never taught to round up or down. I can round up the weight but the exact calculated mg/kg dose is given, no rounding up or down. The common practice where I work is to round the calculated dose up if more than half of the vial and down if less than half is required. This way, no drug is left in the vial and wasted.  Can you please point me to any rounding rules for weight based IV drugs?

A:  Great question, thank you! Let’s review weight based dosing. It is a dosage calculation using the weight of a patient, using either actual body weight or ideal body weight in (kg) and a drug dosage specified in terms of weight, such as (mg/kg).

Here’s an example:

  • A patient’s actual body weight is 135lbs. The patient is to receive “Drug X” at 4mg/kg every 4 weeks.
  • Based on this example, the patient’s weight is in pounds so it has to be converted to kilogram.  1 kg = 2.2 lbs. This patient’s weight in Kg is 61.36kg. (135 divided by 2.2). No need to round weight in this case, simply 61kg.
  • Drug X dosage is 4mg/kg. Pt weight is 61kg. 4mg multiplied by 61kg is 244mg.
  • The weight based dose of the patient is 244mg. (exactly as calculated) 

Again, just as an example, let’s say that each single use vial of Drug X has 100mg/10mL. The patient dose is 244mg. Take 244mg divided by 100mg = 2.44 vials.  This means using 2 full vials of 100mg vials to equal 200mg and 44mg from the 3rd vial. In terms of volume, 20ml for the 200mg from each full vial (10mL /vial) and 4.4mL from the 3rd vial. The patient gets the calculated dose of 244mg (24.4mL) of Drug X and the rest of the drug left in the 3rd vial is wasted (discarded). End of story, right?

Well, usually it is the end of the story unless “Drug X” is one of those expensive IV medications or specialty drugs. This is where that common practice you mentioned, variations (and confusion) of “rounding” of weight based dosing comes into play.  So to answer your question, are there rounding rules for weight based dosing? The theoretical answer is no, what you calculate using the patient’s weight and based on the drug order is the dose you give. However, in clinical practice and as it relates to many of the current expensive drugs in the market, consideration is given to “avoiding drug wastage” by dose rounding. The decision to dose round up or down or give exactly as calculated is at the discretion of the prescriber or based on a dose rounding protocol established by the healthcare organization. None of the drug manufacturers recommend dose rounding in their prescribing information.

When do prescribers decide to round up and when to round down to avoid wastage? This is where the variations occur.  In a national survey by Adler, Sandler, et al. of 279 IBD clinics, 207 responded. Thirty-eight percent (78/207) indicated that their practice has no uniform approach to the rounding of doses. Of 114 respondents indicating a uniform approach to rounding doses, 43% always round up to the nearest 100 mg, 33% always round up or down to the nearest 100 mg, and 14% never round doses.  Wide variations exists, there is no uniform dose rounding recommendations and the effect of these variations on patient outcomes is unknown(Adler et al, 2013).

It is important to note that whatever drug was “wasted” (or discarded) is billed either to the patient’s insurance or to the patient. Medicare defines the discarded drug amount as the amount of a single use vial or other single use package that remains after administering a dose/quantity of the drug to a Medicare beneficiary.  A (billing) modifier must be used in order to obtain payment for a discarded amount of drug in single dose or single use packaging under the Medicare discarded drug policy. Commercial insurers may have the same or different wastage policies. The practice of dose rounding, whether up or down resulted from the financial impact of drug wastage. According to a study on cancer drugs by Bach, Mueller & Saltz, the federal Medicare program and private health insurers waste nearly $3 billion every year paying for medicines that are thrown out because many drug makers distribute the drugs only in vials that hold too much for most patients (Bach et al, 2016). Many insurers advocate for rounding down. According to a study from Magellan Rx Management, the pharmacy benefit management division of Magellan Health Inc, rounding down doses for injectable monoclonal antibodies within 10% to achieve more efficient use of vial sizes could lead to substantial savings for health plans. (Blum, 2017) 

What are the impact of dose rounding up or down on patients?  There are no published articles that I can find specific to dose rounding just anecdotal reports. Fear of rounding up may overdose patients, and underdose/undertreating if rounded down. One can always refer to the studies by the manufacturers for maximum doses and again use the discretion of the prescriber to evaluate dose rounding if patients are not experiencing the expected outcomes.

What are the nursing implications for dose rounding?

  • Understand the rationale and implications behind dose rounding. Many nurses just round up or down without knowing why.
  • Know the patient’s dose and recommended dose from the manufacturer for the disease state. Many nurses think this is the precriber’s and pharmacist’s responsibiities, not theirs.
  • Always check the dose calculations each time the patient comes for an infusion. If given an exact drug dose vs mg/kg – do a reverse calculation by dividing dose with patient’s weight.
  • Don’t assume you can just round up or down because that’s the common practice in your facility or that’s what you were told. The physician orders for dose rounding should be written specifically as do not round, round up or round down. A dose rounding protocol or policy should be established that specifically outlines the drugs eligible for dose rounding and the dose rounding procedure.
  • Weigh the patient each time they come for an infusion to obtain a current actual weight. Not having a scale is not a reason not to weigh patients.
  • Document your calculated drug dose based on the patient’s actual weight, the rounded dose (up or down or none) and drug wasted, if any.
  • Assess patients for improvement, lack of, or potential adverse events and notify the prescriber.
  • Never share left over vials with other patients.


References used are in active links in text.





2 thoughts on “Q&A: Are there rounding rules for weight based dosing?

Comments are closed.