Accuracy

Some medical professionals still consider mercury to be the only accurate and consistent method of measuring temperature and blood pressure. Yet, as peer reviewed studies from the last decade demonstrate, this is not currently the case, and in fact our attitude toward the accuracy of mercury thermometers and sphygmomanometers was probably overly positive in years past as well.

The mercury filled glass thermometer, though easily and frequently broken, is one of the simplest and most widely used diagnostic tools. It was therefore the first clinical mercury device to be evaluated for accuracy in comparison with a growing number of available alternatives.

After considerable debate in the 1990s, Leick-Rude and Bloom, during routine accuracy testing in a study, reported that 25% of the glass/mercury thermometers tested differed from the reference thermometer by >0.2 degrees Centigrade. This finding was consistent with the authors' review of prior work. Indeed, another recent study had rejected 28% of glass/mercury thermometers due to inaccuracy.

Digital thermometers, the most commonly used mercury-free temperature device, use a thermistor to convert temperature into a known electrical resistance, and are highly sensitive. As with most products (mercury or mercury free) their accuracy is dependent on manufacturing quality and techniques. Standards organizations such as the ASTM International have developed protocols that will help the healthcare community identify accurate alternatives. It is imperative that the healthcare community and governments ensure that thermometers are purchased from manufacturers that follow techniques and testing protocols that are independently certified by ASTM or other internationally established regimes, so as to provide a product that provides the accuracy required.

Sphygmomanometers represent the largest reservoir of mercury in current medical use. As with thermometers, mercury and non-mercury blood pressure devices provide accurate measurement so long as both instruments are calibrated. Examples of both inaccurate mercury and mercury-free sphygmomanometers can be found in the medical literature, though this inaccuracy is typically related to poor maintenance and calibration. A large number of scientific studies have concluded that mercury-free measuring devices produce the same degree of accuracy as mercury devices, provided they are properly maintained and calibrated. For instance, a study at the Mayo Clinic in the US concluded that aneroid sphygmomanometers provide accurate pressure measurements when a proper maintenance protocol is followed.

Some have argued that for accurate blood pressure measurement the reference device used for calibration must be a mercury blood pressure device (with a typical error of +/- 3mm of mercury) . Yet, when calibrating a device the error of the reference pressure should be added to the specified accuracy of the instrument under test (+/-3 mm Hg) to determine the working accuracy of a calibration set-up. As a result, if using a manometer (mercury column or aneroid gauge) rated at ±3.0 mm Hg as a reference, one will be able to determine the accuracy of the gauge being tested to only ±6.0 mm Hg. This is outside the range of +/- 5mm of mercury typically desired by medical professionals. Many facilities and device manufacturers are using a device (e.g., digital pressure standard) rated at ±0.1 mm Hg, one will be able to determine the accuracy of the gauge being tested to within ±3.1 mm Hg. This has been well documented to be far less than the inter or intra provider differences on multiple readings.

A U.S. study from 2003 concluded in summary that "research on sphygmomanometers suggests that there are numerous good alternatives to mercury sphygmomanometers. Aneroid sphygmomanometers are cost competitive, have a long history in the field, and have been found acceptable by many hospitals."

In a UK study, an aneroid device achieved an A grade for both systolic and diastolic pressures and fulfilled the requirements of the Association for the Advancement of Medical Instrumentation. The conclusion was that the aneroid device could be recommended for use in an adult population.

The UK Medicines and Healthcare Products Regulatory Agency (MHRA) states that aneroid and mercury sphygmomanometers both need to be checked regularly in order to avoid errors in blood pressure measurement; the British Hypertension Society recommends testing every 6 to 12 months.

Frequently lost in the discussion over device accuracy, and equally important is the issue of measurement technique. A 2002 Working Meeting on blood pressure measurement in the United States highlighted numerous studies which found that basic measurement technique, inappropriate cuff size and poor cuff size were providing significant errors in measurement.

Switching to mercury free sphygmomanometers in clinical settings has not caused problems in clinical diagnosis and monitoring in Sweden or Brazil. The Swedish government, in fact, has completely eliminated mercury column sphygmomanometers.

After considering the scientific evidence, a report produced by the World Health Organization (WHO) department addressing cardiovascular diseases concluded in 2005 that even in low resource settings, "in light of the toxicity of mercury, it is recommended that mercury blood pressure measuring devices be gradually phased out in favour of affordable, validated, professional electronic devices."

One problem that several hospitals in developing countries have encountered as they substitute mercury-containing sphygmomanometers is that many aneroid and digital devices are of poor quality yet many devices currently produced satisfy the criteria of professional organizations such as the British Hypertension Society, the European Hypertension Society and the Association for the Advancement of Medical Instrumentation. The British Hypertension Society (BHS) has created a list of vendors of sphygmomanometers that have met the BHS criteria and are suitable for clinical practice and posted it on their web site .

The above cited WHO report points out that international protocols for blood pressure measuring device validation have been released by the Association for the Advancement of Medical Instrumentation, the British Hypertension Society, and the European Society of Hypertension Working Group on Blood Pressure Measurement.

Affordability

Many healthcare practitioners are concerned about the availability of alternatives. In fact, there are many mercury-free thermometers and sphygmomanometers available from major medical equipment suppliers who service the global market.

Yet the issue of affordability is still a challenging one, especially where the costs of human and environmental impacts of mercury releases are not included in the health institution's accounts or budgets. From a developing economy's perspective, these costs must be taken into account in national strategic planning.

In countries such as the United States, where market demand for mercury alternatives has begun to be felt and clean up costs quantitated a mercury free purchasing policy has become the most economic. In a study done by Kaiser Permanente, the largest not-for-profit Health Maintenance Organization (HMO) in the United States, it was determined that when associated lifecycle costs are included (compliance, liability, training, etc.) the total cost per unit of an aneroid sphygmomanometer is about 1/3 that of a mercury-containing device. Mercury-containing devices are no longer being purchased by Kaiser Permanente.

Yet in the global market, mercury-based medical devices are still significantly less expensive than their digital or aneroid counterparts. In the absence of strict environmental health regulations, and with limited healthcare budgets, many health care systems and hospitals today still face the challenge of deciding between a mercury device and its alternative. Those facilities with limited budgets have been able to successfully avoid this road block through operational strategies.

For example, when planning future budgets hospitals are counting the frequent mercury thermometer breakages for inclusion in the cost of current practices for comparison with the cost of a digital or mercury-free alternative. Frequently, the additive cost is comparable to the replacement cost of the mercury thermometers, as the alternatives are typically more durable.