‘Cultural metrication’ and the NHS

A reader of Metric Views from the USA raises the issue of measurement units in medical practice generally, or ‘cultural metrication’ as he terms it.

An article in Metric Views in December 2009 discussed the LACORS survey on weighing scales in NHS hospitals, http://tinyurl.com/yao9kzb . This issue was also taken up on 25 February in Parliament by Lord Walton, http://tinyurl.com/y8m45z3.

Paul Trusten writes from the US:

At the risk of being called a meddling Yank, my answer to the question offered here – namely, should a totally metric culture be enforced by NHS – I say, emphatically and categorically YES!

The use of two different systems of measurement in healthcare must end. Only the SI metric system should be used in medical records and in verbal communications of a patient’s weight and height. 

I say this not just out of my commitment to metrication on my side of the pond as the Public Relations Director of the U.S. Metric Association, but, much more importantly, as a practicing pharmacist for the past 33 years. I agree completely with the statement that the use of customary units alongside metric units for height and weight in a hospital is a dangerous practice that should be ended as soon as possible, on both sides of the Atlantic.  The entire culture in hospitals must be metric, and conversion of any kind should be banned.  Ideally, weigh patients on scales that read in kilograms only, and measure height in meters or centimeters only.  I believe that the traditional romance of weighing babies in pounds and ounces must be exchanged for the peace of mind parents should enjoy from the enforcement a metric-only healthcare environment.  From a safety standpoint, their precious bundle’s weight in kilograms should be music to their ears.

My commitment to the goal of U.S. metrication began in 1974 when I was in pharmacy school.  When I was studying pharmaceutical calculations, I was horrified to learn that two, even three systems of measurement (apothecary, avoirdupois, and metric) were involved.  (Editor: It became illegal in the UK from 3 March 1969 to use any system of weights and measures other than the metric system for dispensing prescriptions).  Also, the decimal nature of the metric units made them so much simpler to manipulate mathematically.  Anyone who has ever tried to add apothecary weights denominated in ounces, drams, scruples, and grains vs. just adding up figures in grams will understand. My quest for one system of measurement in healthcare was easily extended to the goal of metric measurement for all purposes.  It just made sense.

Banish any thought that medication dosing is ever going to accommodate the old units by being stated in something like milligrams per pound. That is out of the question.  Medications can only be handled in the decimal metric system. Period. In the preparation of compounded sterile injectable products (intravenous and other injectables), use of only the decimal metric system is possible. Decimal calculations are the only safe ones.

For certain age groups (usually pediatric patients), medications are dosed on a milligram per kilogram of body weight basis.  The anticoagulant drug heparin is often dosed at the rate of units of heparin per kilogram per hour. Many drugs, especially anti-neoplastic agents (chemotherapy or anti-cancer drugs),  some antibiotics,  and other specialized drugs, are often dosed in micrograms or milligrams per square meter of body surface area (BSA), with BSA being calculated using the patient height and weight in metric units.  If, in the dose calculation, the patient weight in pounds were to be accidentally substituted for the weight in kilograms, a disastrous overdose could occur.  In this arena, well will not have metric martyrs. We will have customary ones.

U.S. healthcare safety officials made considerable progress in cultural metrication last year.  The Institute for Safe Medication Practices (ISMP),  a non-profit organization devoted entirely to medication error prevention and safe medication use, has recommended the total metrication of the medication delivery process, from prescribing to order entry to pharmacist counseling of the patient (see http://www.ismp.org/Newsletters/ambulatory/archives/200905_1.asp).

I want to suggest also that body temperature should be taken and recorded only in degrees Celsius, to allow for worldwide ease of communication. 

Be totally safe – be totally metric!  Let us all, UK and U.S., at long last, use only one system of measurement in healthcare.

19 thoughts on “‘Cultural metrication’ and the NHS”

  1. In order for the NHS to go completely SI, it will require more then just assuring all the scales read in kilograms only. Hospital staff must be required to have a working knowledge of SI units and must speak only in SI while on the job, both amonst themselves and to patients.

    When hospital staff are weighing patients, it doesn’t help the situation if they have one eye on the scale and the other on a conversion chart. It defeats the purpose of the metric only scale if they convert measures either via charts or by other means and refer to the converted results rather then the measured from that point on.

    Experience, competence and a feeling for metric numbers can only come from daily use. If hospital staff convert then they will never obtain the needed familiarity with metric units and will be more prone to mistakes. Thus dosing errors will continue as will misplaced decimal points, as the errors will never be noticed due to lack of familiarity with the metric system.

    Recording data in records in both SI and non-SI units must also stop and only SI must be used. Reading the wrong numbers on the record can be just as dangerous as making a wrong calculation.

    Assuring that the language of the hospital is always SI is as important as making sure all the instruments are SI.


  2. So, then why hasn’t anyone from BWMA been protesting all these years that using “grains” instead of “milligrams” is more “natural” for patients? Same story for keeping the old L-s-d money (doesn’t 12 have more integral divisors than 10)?

    I still shake my head at the incredibly selective nature of their protests. But, of course, the government’s unwillingness to complete metrication is the real crime since most of the British public doesn’t seem likely to rise up en masse and erect barricades in protest.


  3. While I am satisfied that British hospitals use metric units as a matter of routine, there is possibly a culture of “We will use metric units to keep the auditors happy, but will otherwise use imperial units”.

    Apart from making medically-oriented articles unintelligible for the man in the street (unless the units are dumbed down), there is also the risk that medical staff who think in terms of imperial units in respect of their personal lives will be slow to pick up errors (such as recording 572 kg instead of 57.2 kg) because “these are just meaningless numbers”.

    There is also the risk of a figure-conscious nurse switching scales to show imperial units so that she can check her own weight, and then forgetting to reset them, resulting in a person’s weight being recorded as 128 kg rather than 80 kg (12 st 8 lbs) Following recent government announcements, this might well become a thing of the past due to dual-unit scales being prohibited in hospitals.


  4. The LACORS web site has a page dated 3 Nov 2008 (http://www.lacors.gov.uk/lacors/NewsArticleDetails.aspx?id=20484) which describes the problems they found with weighing machines in hospitals, then mentions a repeat inspection to be held in spring 2009 followed by a “final report to be produced on the change that the project has helped bring about”.

    I can’t seem to find anything about this repeat inspection much less the final report. Does UKMA have any information on these items?


  5. A friend remarked to me yesterday that as recently as Friday morning she was told during a doctors appointment that she had “lost 3 pounds since her last visit”, the scales used to weigh here being visibly dual unit and no attempt was made to mention any weight in kg. This is a person who relies on a number of medications and actually does use metric scales at home!

    It seems clear that there is currently no effort being made to stamp out this potentially life-threatening practice; it seems crazy to me that when so many professionals (both medical and otherwise) recognise the dangers that nobody is willing to tackle the issue head on. While we all realise that politicians see metrication as a political hot potato it’s criminal that they’re so unwilling to act even when public safety is concerned!


  6. Given Alex’s comment, I’m wondering how doctors and nurses outside of hospitals could be convinced to use only metric scales and record all data in metric. Logically, it would seem to me that everyone associated with the NHS, whether working in a hospital or not, should be doing this.


  7. No one has mentioned blood pressure. It is universally measured in the NHS using mmHg (mm of mercury). This is not an SI unit. Since I suffer from hypertension and regularly monitor my own blood pressure I have got used to the unit. In fact I’ve not even worked out the equivalent in Pa, which is uncharacteristically lax of me. It would be interesting to hear if anyone has come across any different blood pressure units being used in medicine.


  8. Along the lines of what David Brown mentions, I have been using a CPAP machine (Continuous Positive Airway Pressure) for over 10 years to treat my sleep apnea (quite effectively, I might add).

    The weird thing is that the air pressure is measured in terms of the pressure needed to maintain a column of water x cm high. So, in my case the machine is calibrated for “13 cm of H2O”).

    Does the NHS measure CPAP pressure in this same bizarre fashion? I don’t even know how this got started (though I understand the first CPAP machine was invented in Australia back in the 70’s).

    In any case the goal should clearly be for all personnel in the medical professions to use true SI exclusively. Let’s hope that can be accomplished soon.


  9. I disagree entirely with the premise that we should abandon Imperial for Metric. Anybody who can only work in 10’s shouldn’t be trusted to do my medicine!
    Working on an elderly acute ward I am continually asked to work out stones and pounds, feet and inches for people who will never wish to go metric. Why should we be compelled to? My experience of drug errors is of mathematically incompetent nurses using metric measures.


  10. 1. Would Ralph please give an example (showing working) of a drug dose calculation using only imperial units.

    2. Is Ralph suggesting that all nurses choose the measurement system they want to work in? If this is the case he is encouraging some nurses to do conversions between systems.

    3. Does Ralph also spend time doing money calculations using Pounds, Shillings, and Pence for people in his care?


  11. Ralph alleges that some nurses he knows are not good at maths, and that some patients cannot do the arithmetic needed to convert between different units.

    If ever a case was needed for not returning to the dark ages of mathematically-complicated and error-prone imperial units in medical dose calculations then this is it.

    It would be quite reckless to increase the potential for errors in anything to do with medical use. Fortunately, medicine in the UK has been 100% metric for decades.


  12. To Ralph

    You raise a number of issues but I will deal with just one of them as my response to it is a bit lengthy.

    If the UK goverment had handled the changeover to metric, announced in 1965, properly and completed the project within 10 years according to their stated intention, then the people now in your care, who would have been younger and more adaptable at the time, would have understood the advantages of us using the metric system and the reasons for the change. By now they would have been happy to be weighed in kilograms just as they count their money in (new) pence instead of shillings and old pence.

    Alas it didn’t happen that way. The government never bothered to promote the change or educate people accordingly. Worse still they betrayed the whole process and sent out the wrong message by not changing road signs, a major element directly under their control. They did instigate changes in health, education and other public service practices but only because there was no short term impact on the chancellor’s budget.

    However, it is not too late to put this right. It is not realistic to expect the people in your care to change belatedly – true enough – and we can sympathise with the situation you now face, but at the same time we shouldn’t condemn future generations to the same fate.

    As it is, being stuck half-way through the change, we all have to cope with both imperial and metric which, as you have experienced directly, is not easy or convenient and certainly not necessary.

    The only realistic way out of this enpasse is to complete the changeover and phase out imperial measures decisively.


  13. The Institute for Safe Medical Practices (USA) has once again (as of Oct 2011) called for metric only dosages for OTC (over the counter) medicines:


    Let’s hope we here in the USA finally take at least this step towards better measurement.

    I presume the NHS, chemists, etc. all use metric for dosages including printed inserts and directions on packaging. Is the conversion to metric-only weighing scales being pursued as well?


  14. Ralph Hulbert said:

    “Working on an elderly acute ward I am continually asked to work out stones and pounds, feet and inches for people who will never wish to go metric.”

    What exactly needs to be worked out? Drug doses are worked out by the doctors and are based on metric amounts, including body mass in kilograms. The patients, no matter what their age, don’t need to know the details of how the dose is derived.

    “Why should we be compelled to? My experience of drug errors is of mathematically incompetent nurses using metric measures.”

    You should be compelled to because that is the agreed standard for the industry. If you don’t like it then resign from the medical industry.

    If the nurses are incompetent using metric measures, why are they still employed? Aren’t they reviewed? Doesn’t someone check to make sure medical professional are fully capable of doing their job? If they can’t do the math in metric, there is no way they would be able to do the more complex math using imperial.

    On the flip side of the coin, are they really incompetent in using metric or are they just unable or don’t wish to bother with metric to imperial conversions, pushing the burden on you? Which is it?


  15. @David Brown

    No one ever answered your question about blood pressure. In the US, we also use mm Hg. However, Continental Europe mostly uses the kilopascal.

    Converting “real” columns of mercury to pascals involves latitude and height above sea level (for local gravity) and temperature for thermal expansion of both the mercury and the brass scale. However, “conventional” mercury is based on 760 mm Hg = 101.325 kPa, regardless of gravity and temperature; aneroid instruments are usually calibrated on this basis. A VERY close approximation, easier for mental conversion is 750 mm = 100 kPa.
    120/80 (mm Hg) becomes 16/10.7 (kPa). Hectopascals would provide better resolution (using integers) but the possibility of a units mistake if units are omitted as is the norm.

    If the nurses can’t work out metric dosing, grains or av. scruples per stone/pound must just be a delight. And after working that out, you still have to consider liquid medicine in grains/fluid drachm and find a suitable syringe. And, of course, you mustn’t confuse your apoth scruples and fluid scruples, drams, etc.


  16. To minimise the chance of errors there needs to be some effort to minimise the use of decimals in prescribing drugs. There have been cases of milligrams being mixed up with micrograms, so the best solution might be to express all dosages in micrograms. Then, instead of having 0.7 milligrams it would be written as 700 micrograms. If this was done consistently it would help to reduce medical errors.


  17. The use of dual measurements in health care introduces risks with conversion errors. That is how NASA lost the Mars Climate Orbiter. Mistakes in health care can lead to really serious consequences. The obvious solution is to use a single measurement system. The question that arises is, “Which one?”. The best one should be used. Remember that the imperial system is no good to describing medical doses, which are normally expressed in milligrams or micrograms. It is also no good for expressing the chemical compounds of a bottle of mineral water because of its limited range. Also, there are no imperial units for electricity or radiation. Thus the metric system can be used for all health care needs whereas the imperial system cannot.


  18. The Agency for Healthcare Research and Quality very recently posted a case about a child whose weight was recorded incorrectly as 55 kg instead of 55 lb and given too high a dose of medication:


    It is high time we in the States switched entirely to metric in the medical arena. I certainly hope it is the case that NHS is 100% metric these days (other than converting new-born babies’ weight to pounds “for the parents” as I understand it).


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