Spoons Systematically Bias Dosing of Liquid Medicine

Background: Spoon dosing has been identified as a major cause
of dosing errors and pediatric poisonings (1). Although the U.S.
Food and Drug Administration recommends against using kitchen
utensils to dose liquid medicine (2), most persons still use spoons
when pouring medicine for themselves and their families (3). Although
dosing errors remain modest when using kitchen teaspoons,
they may increase when using various sizes of larger spoons (4). If the
size of a spoon leads a teaspoonful of liquid medicine to seem like
markedly more or less than 5 mL (5), a person may compensate by
under- or overdosing (Figure).

Objective: To examine whether the dose of liquid medicine varies
depending on the size of the spoon onto which it is poured.

Methods: During the cold and flu season, we asked 195 university
students (109 men; mean age, 20.1 years [SD, 1.7]) who were
recent patients at a university health clinic to dose 5 mL of cold
medicine into a teaspoon (5 mL, 2.7 x 4 cm), a medium-sized tablespoon
(15 mL, 4 x 6 cm), and a larger spoon (45 mL, 6 x 9 cm).
We told them that they were participating in a study about cold
medicine and asked them to suppose they were at home with a cold,
taking liquid medicine with a recommended dose of 1 teaspoon. So
participants would better understand the volume of a teaspoon, we
first gave them a full bottle of cold medicine and a teaspoon and
asked them to pour exactly 1 teaspoon (5 mL). Next, we asked
participants to pour the same 5-mL dose into each of the remaining
2 spoons in a randomized order. After each of these 2 pours, we
asked participants to indicate how confident they felt that they had
poured 5 mL (1 = not very confident; 9 = very confident) and how
effictive they believed their poured dose would be (1 = not very
effective; 9 = very effective). After they left the room, we measured
the volume of cold medicine they had poured into each of the 2
spoons. We performed all analyses by using SPSS statistical software,
version 15.0 (SPSS, Chicago, Illinois). We considered a P value less
than 0.050 to be statistically significant.

Results: The amount of cold medicine that participants poured
varied directly with the size of the spoon (4.58 vs. 5.58 mL; t =
4.63; P < 0.001). Participants underdosed when using the mediumsized
spoon (4.58 vs. 5 mL; t = <2.30; P = 0.022) and overdosed
when using the larger spoon (5.58 vs. 5 mL; t = 2.39; P = 0.017).
Although the capacity of the spoons was never a constraint,
participants dosed 8.4% less than prescribed into the medium-sized
spoon and 11.6% more into the larger spoon. Notwithstanding this
aggregate bias of 20%, participants had above-average confidence
that their pouring was accurate and believed that the doses they
poured into both spoons would be equally effective.

Discussion: The amount of liquid medicine a person doses may
vary with the size of the spoon used. Participants underdosed by
8.4% when using medium-sized spoons and overdosed by 11.6%
when using larger spoons. Although these educated participants had
poured in a well-lit room after a practice pour, they were unaware of
these biases and were confident that they had poured the correct
doses in both spoons. Whereas the clinical implications of an 8% to
12% dosing error in a 1-tsp serving of medicine may be minimal, the
dosing error is likely to accumulate among fatigued patients who are
medicating themselves every 4 to 8 hours for several days.
Although one would expect more experienced pourers, such as
nurses or practiced parents, to be less biased, this may not be so.
Even confident veteran bartenders poured 28% more liquor
into short, wide glasses than into tall, slender glasses of the same
volume (6). If a medicine’s efficacy is tied to its dose, it is more
effective to strongly encourage a patient to use a measuring cap,
dosing spoon, measuring dropper, or dosing syringe than to assume
that they can rely on their pouring experience and estimation abilities
with kitchen spoons.

Brian Wansink, PhD
Koert van Ittersum, PhD
Cornell University
Ithaca, NY 14853

Potential Conflicts of Interest: None disclosed.

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