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27 April 2010
This Australian observational study found that blood pressure (BP) measured in clinic by specialist staff using immaculate technique was higher than ambulatory BP values by an average of 6/3mmHg. BP measured in clinic by referring doctors was higher still, by an average of 9/7mmHg compared with readings taken by specialist staff. The study also estimated ambulatory BP thresholds equivalent to clinic BP measured using immaculate technique.
Level of evidence:
Level 3 (other evidence) according to the SORT criteria.
Action
This study highlights the need for immaculate technique when measuring BP in clinic: it suggests that clinic measurements made by doctors were substantially higher than ‘gold standard’ clinic measurements made by specialist nurses. If confirmed, this could have profound implications for diagnosis and treatment. The suggested ambulatory BP thresholds and targets equivalent to targets in guidelines for clinic-measured blood pressure may be useful for specialists. However, NICE guidance does not recommend routine use of automated ambulatory BP monitoring (or home monitoring devices) in primary care, because their value has not been adequately established.
What is the background to this?
Measuring BP accurately is crucial to effective diagnosis and treatment. NICE guidance provides thresholds for initiating BP lowering treatment, and targets for which to aim. However, these are derived from studies which used clinic measurement. As the accompanying editorial says, diagnostic and treatment thresholds for ambulatory measurements in international guidelines vary considerably, despite near unanimous agreement for clinic thresholds. Better agreement on ambulatory thresholds would allow comparisons between countries and reduce confusion for clinicians.
An earlier study had found that BP measured in clinic by doctors was higher than that measured in clinic by nurses, and both were higher than ambulatory BP: the so-called ‘white coat’ effect. In addition, and as discussed on the hypertension section of NPC, there is evidence that BP measurement in clinic is not always done with immaculate technique (see especially part 2 of the <60minute eLearning event).
This study set out to compare measurements of clinic blood pressure and 24 hour ambulatory blood pressure in 8,529 patients referred to 11 hypertension clinics across Australia. The aim was to relate the results from these two types of measurement and so produce ambulatory BP thresholds and targets equivalent to the clinic thresholds and targets given in clinical guidelines. In addition, datasets from four centres included the referring doctor’s clinic blood pressure measurements, and these were also compared with the specialist clinic measurements and the ambulatory measurements.
In the majority of instances, clinic blood pressure was measured by research nurses and research staff trained in the measurement of blood pressure. Clinic blood pressure was measured after a 10 minute rest using an appropriately sized cuff and a mercury sphygmomanometer in all but three minor contributing centres total 7.3% of the data), where a digital device was used. The average number of clinic measurements across the 11 centres was 2.8 readings with an average of 2.4 readings used per person if the initial measurement was excluded.
What does this study claim?
A secondary finding of this study is likely to be of most practical value to most primary care clinicians. After adjusting for age, sex and treatment, specialist clinic BP measurements made using immaculate technique were on average 9/7mmHg lower than clinic measurements made by referring doctors.
Average BPs measured in the clinic by specialists using immaculate technique were 6/3mmHg higher than daytime ambulatory BP and 10/5mmHg higher than 24 hour ambulatory BP. However, the closer a patient’s BP was to ‘normal’ BP, the closer the ambulatory BP was to the clinic BP. For example, if the clinic BP was 125/75mmHg, the day ambulatory BP equivalent (predicted by least product regression analysis) was 124/74mmHg, and 24 hour equivalent BP was 121/71mmHg. Conversely, the higher a patient’s BP, the greater the difference between clinic and ambulatory BP. For example, for clinic BP of 180/110mmHg, the day ambulatory BP equivalent was 168/105mmHg, and 24 hour equivalent BP was 163/101mmHg.
So what?
This study confirms that ambulatory BP measurements are likely to be lower than clinic measurements. The most important messages from this study are firstly that repeated measurements by trained staff using immaculate technique can narrow the difference, especially at lower blood pressures. Secondly, the difference between BP measured in clinic by referring doctors and BP measured in clinic by specialist nurses under ideal conditions was greater than the difference between the latter and ambulatory BP measurements.
As NICE guidance states, healthcare professionals taking blood pressure measurements need adequate initial training and periodic review of their performance. Devices for measuring blood pressure must be properly validated, maintained and regularly recalibrated according to manufacturers’ instructions.
To identify hypertension (persistent raised BP above 140/90mmHg), patients should normally have their BP measured under the best conditions available at least six times over three clinic visits a month apart. (Patients with accelerated [malignant] hypertension [BP more than 180/110 mmHg with signs of papilloedema and/or retinal haemorrhage] or suspected phaeochromocytoma should be refered immediately).
NICE guidance does not recommend routine use of automated ambulatory BP monitoring in primary care, because its value has not been adequately established. Epidemiological studies are inconsistent in demonstrating the additional prognostic value of ambulatory blood pressure monitoring to predict cardiovascular disease in unselected patients
Design: Observational study
Patients: 8,575 patients with hypertension referred to 11 centres across six Australian states. Mean age of participants was 56 years with mean body mass index 28.9kg/m2 and mean clinic systolic/diastolic blood pressure 142/82mmHg. 54% were women.
Intervention and comparison Least product regression to assess the relation between ambulatory blood pressure measurements and clinic blood pressure measured by trained staff and in a smaller cohort by doctors (four centres, n=1,693).
Outcomes and results Average clinic measurements by trained staff were 6/3mmHg higher than daytime ambulatory blood pressure and 10/5mmHg higher than 24 hour blood pressure, but 9/7mmHg lower than clinic values measured by doctors. Daytime ambulatory equivalents derived from trained staff clinic measurements were 4/3mmHg less than the 140/90mmHg clinic threshold, 2/2mmHg less than the 130/80mmHg threshold, and 1/1mmHg less than the 125/75mmHg threshold. Equivalents were 1/2mmHg lower for women and 3/1mmHg lower in older people compared with the combined group.
Sponsorship Financial sponsorship was supplied to the first author for the analysis of the data from the High Blood Pressure Research Council of Australia. All authors are members of the council.
More information on hypertension can be found on the hypertension section of NPC
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