The "penny in a cup" test offers general information about a freezer's temperature, but it lacks the necessary sensitivity to determine whether frozen medications are kept in the desired temperature range.
Refrigeration in hospital pharmacies is important for maintaining the integrity of the medications to be dispensed within those pharmacies. Frozen medications must be kept at a temperature of —25° to –10°C, according to the USP <797> guidelines.1 Many hospital pharmacies use thermometers to test their refrigeration, but others use something a little less accurate.
The “penny in the cup” test is often used in hospital pharmacies as a potential test for thawing when periodic checking is not an option.2,3 The test itself is quite simple. It involves taking a small cup and filling it partially with water. The water is then frozen. A penny or paperclip is placed on top of the frozen water and the cup is placed into the freezer, which is located in the refrigerator being tested, usually overnight or during a break when employees will not have the opportunity to check the thermometers. When the employees return, they check to see if the penny is still on top of the ice. If it is frozen inside the ice, this may indicate there was an extended thaw during the time in question.4
Because water is so important in this test, a review of one of its physical properties is in order. The freezing point of water is 0°C.5 As indicated previously, USP <797> mandates that frozen medications must be maintained between —25° and –10°C. This means any temperature between –9° and 0°C is inadequate, but the penny test will not indicate any problem.1 The penny will still be on top of the ice. Furthermore, many vaccines that require frozen temperatures actually have storage indications that are at most —15°C. This means that if the temperature in the freezer is between –14° and 0°C, the water in the cup would remain frozen but the temperature would not be adequate for the stability of these drugs. Many vaccines, such as the chicken pox, shingles, and MMR vaccines, fit into this category6 (Table).
Compounded sterile preparations (CSPs) that should be frozen can also pose a problem. Low-, medium-, and high-risk CSPs all are allowed a beyond-use date (BUD) of 45 days if frozen. USP indicates this temperature as being lower than —20°C, which allows errors to occur between –19 and 0°C if the penny in the cup test is utilized.7
The variability within many freezers can also be a source of potentially inadequate temperatures. The temperatures tested in certain upright freezers were found to be higher near the front and the top of the freezer.8 Therefore, the location of the cup when it is placed into the freezer can also be a source of inaccurate conclusions regarding the efficiency of the freezer in question.
Problems arising from inadequately controlled storage of drugs are diverse. The aforementioned vaccines have the potential to cause increased drug interactions when exposed to these temperatures, such as a patient that developed painful vesicles on their forearm when given a varicella vaccine that was not maintained at an appropriate temperature.9 The drugs themselves, particularly frozen vaccines, are only supposed to be exposed to refrigerator temperatures (2° to 8°C) for up to 72 hours, usually during transportation of the drug. Also, the manufacturers of the vaccines recommend discarding the drug if it has been exposed to temperatures higher than —15°C for longer than 72 hours.10 This means that frozen vaccines exposed to temperatures higher than —15°C but lower than the freezing point of water (0°C) for longer than 72 hours should not be used and should be discarded (Online Figure1).
The Joint Commission (TJC) indicated in a 2008 publication that this test does not meet the standards set forth, and therefore should not be utilized. Numerous reasons are indicated. First of all, the penny in the cup test does not have the required sensitivity regarding frozen medications. The test will not detect temperatures below a desired range, and the penny could potentially not sink even though temperatures are above those indicated. Variability is also a problem. The freezer can have large amounts of collected ice that can insulate the ice in the cup even though temperatures are higher than freezing, and well-insulated freezers could also protect the ice without protecting the medications within. Lastly, the test is obsolete because of the many thermometers and alarms available today. Therefore, the test is not as adequate or accurate as the other choices for hospitals.4
In conclusion, it is easy to assume tests such as the “penny in the cup” test will accurately inform health care workers about problems in their freezer. But as TJC has indicated, it is not adequate and should not be used when other, much more informative possibilities exist today.
Gabriel E. Pray is a PharmD candidate at Southwestern Oklahoma State University College of Pharmacy. Singh Saluja PhD, is an assistant professor of pharmaceutical sciences at Southwestern Oklahoma State University College of Pharmacy.