Editorial: Quality Assurance in Compounding, Part IV

United States Pharmacopeia (USP) Chapter <1176> Prescription Balances and Volumetric Apparatus

This week we will look at equipment used to measure liquids, including graduated cylinders, conical graduates, pipets, micropipets, and droppers.

Both graduated cylinders and conical graduates are commonly used to measure liquids. Cylinders are more accurate and should be used preferentially over conical graduates. Even though conical graduates are easier to mix in and clean, the error associated with their use is larger than when a cylinder is used. When observing the graduations on the side of the graduate, an error of 1 mm in reading is relatively constant in a cylinder but the error increases as a bigger volume is measured in a conical graduate. For example, in a 100 mL graduated cylinder, the error is about 0.5 mL but in a conical graduate it could be 1.8 mL. In pharmacy compounding, conical graduates having a capacity of less than 25 mL should not be used.

It is best to select a graduate to use that has a capacity equal to or just exceeding the volume to be measured. Generally, it is best to not measure a quantity less than about
20-25% of the total volume of the graduate.

When measuring small volumes, it is best to use a pipet or even a micropipet. Pipets are available in different sizes and the same general rule applies for selecting which one to use. It is best to determine whether or not the pipet is a TC (to contain) or a TD (to deliver). A TC pipet will need to be "blown out" to deliver the entire quantity; A TD pipet will allow the correct quantity to drain from the pipet. Pipets can be either volumetric where they contain a set volume when filled to a mark, or can be graduated to deliver different quantities. Special pipet washers can be obtained to make sure they are properly cleaned and finally rinsed with distilled water. Pipet bulbs should be used to assist in filling the pipet.

If measuring volumes less than a milliliter, micropipets can be used. There are many types available, including fixed and variable. They generally have disposable tips to prevent cross contamination. One can generally cover a range of from about 10 microliters to 1 milliliter with two variable micropipets. Smaller volumes are also available. Micropipets are especially valuable for measuring small quantities or aliquots of potent drugs and for measuring flavoring agents, etc.

Droppers must be individually calibrated for the liquid being dispensed. A dropper meeting USP standards will deliver 20 drops of water at 15� C. It may also deliver up to 50 drops of alcohol; consequently, droppers must be calibrated for each liquid being measured.

This Regulatory Update has been provided by the International Academy of Compounding Pharmacists (IACP). For more information, www.iacprx.org.

Whether their prescriptions are for sterile or nonsterile preparations, get your patients and physicians involved as advocates for pharmacy compounding. Have them learn more by visiting the Patients and Professionals for Customized Care (P2C2) page of IACP at http://www.iacprx.org/site/PageServer?pagename=P2C2_Recruitment

Funding Available for Compounded Medication Studies
The International Academy of Compounding Pharmacists Foundation is accepting applications for several types of studies involving compounded medications. If you are interested, please contact Lizzie Dragon at the IACP Foundation office 281-704-5090 for an application.

To learn more about the IACP Foundation please visit their webpage at http://www.iacprx.org/site/PageServer?pagename=Foundation_about

Check out the dermatomes chart available to help treat pain patients at http://www.ijpc.com/Products/ProductDescription.cfm?PID=2466

Visit the American Society of Health-System Pharmacists drug shortages page at http://www.ashp.org/s_ashp/sec_drug_shortages.asp?CID=1500&DID=1544

Measuring Device Evaluation
To quickly confirm the operation of volumetric equipment, place the device on an electronic balance and tare the balance to zero. Add the desired quantity of purified water to the graduate or device, for example 25 mL. Place the device containing the water on the electronic balance and it should weigh 25 g. Different volumes can be used to confirm the performance of the measuring device.

For micropipets, place a weigh boat on the electronic balance and tare it. Measure a quantity of purified water using the micropipet, such as 200 microliters. Deliver the water to the weigh boat; it should weigh 200 mg.