The Pharmaceutics and Compounding Laboratory
Accuracy in Measurements

Calibrated Medicine Droppers

Calibrated medicine droppers are often used to deliver small doses of liquid medication, 1 ml or less, to the patient. Precision and accuracy of dose from droppers can be very poor. Problems arise from the false assumption that a close relationship exists between a drop and the volume of any liquid. The size of a drop of any liquid will vary not only with differences in the construction and composition of the dropper, but also with the viscosity, surface tension, and density of the liquid. Personal factors also contribute to the problem. Two individuals dispensing the same liquid from identical droppers may produce drops of different sizes because of variations in the pressure, speed of dropping, and the angle at which the dropper is held.

In an effort so standardize the dropper, the United States Pharmacopoeia/National Formulary (USP/NF) has recognized an official medicine dropper which is designed to deliver 1 ml of water in 20 drops with an allowable 10% deviation (± 2 drops). This stipulation does not appreciable improve the problem.

Some commercially produced medications are packaged with a marked dropper which as has been calibrated with that preparation. In order to maximize accuracy of dosing of a compounded solution from a medicine dropper, the pharmacist must calibrate the dropper with the intended solution at the time of delivery.