Know your dose

 

A woman was recently admitted to the hospital. During her admission, a pharmacist asked her what medicines she took at home. The woman stated that she takes methotrexate injections to treat her psoriasis (a skin condition). She stated that she draws the methotrexate “up to the 10” mark on the syringe, but she did not know the dose in milligrams (mg). The woman uses vials of the medicine that contain methotrexate 25 mg/mL. So, the pharmacist thought, if the woman drew the medicine up to the “10” mark on a syringe, the dose must be 10 milliliters (mL) or 250 mg.

The pharmacist knew that this is a really high dose of methotrexate. Typically, a dose of 10 to 25 mg of methotrexate is given once a week to treat psoriasis. So the pharmacist

considered the possibility that the patient was using an insulin syringe to draw up her methotrexate injections. Insulin syringes are small syringes that have markings for whole numbers including a “10” that measures the medicine in units, not mL. This was also concerning because “10 units” drawn into an insulin syringe would have equaled only 0.1 mL or 2.5 mg of methotrexate, which would have been a low dose.   

The pharmacist contacted the retail pharmacy that provided the woman with the methotrexate she takes at home. The pharmacy stated that they had dispensed tuberculin (TB) syringes to the woman to use to draw up her dose of methotrexate. Some TB syringes use a trailing zero after the decimal point (i.e., 1.0 for 1 mL) which can look like the number 10 if the decimal point is missed (Figure 1).

tb syringe

Figure 1. TB syringe with error-prone measurement marks that include a dangerous trailing zero (i.e., 1.0).

Fortunately, the woman had been drawing up the correct 1 mL amount (25 mg) but thought she was drawing the medicine up to the “10” mark on the syringe. She did not see the decimal point in the “1.0” syringe marking. This could have resulted in a serious medication error if the pharmacist had entered “10 mL” into the woman’s health record, or if she had indeed been taking the wrong dose. The pharmacist was able to educate the woman about the correct dose of her medicine.

Here’s what you can do: Ask your pharmacist about your medicine. Learn what the correct dose is in mg (or units for insulin). For medicine that needs to be injected, learn how to properly measure the correct dose in the syringe provided. Practice drawing up the medicine in front of the pharmacist. Write down the medicine name, dose, device used (i.e., syringe type), and what marking the medicine should be drawn up to on the device.

Created on February 16, 2022

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