How to reduce errors in blood pressure measurement and improving the reliability of proteinuria estimate using dipstick testing

How to reduce errors in blood pressure measurement

The following guidelines were prepared by Professor Andrew Shennan from the Maternal and Fetal Research Unit at St Thomas’ Hospital. Professor Shennan directs the MFRU Blood Pressure Measurement Group for the validation of new blood pressure devices to the British Hypertension Society and AAMI Standards.

The guidelines were initially produced for inclusion in the Pre-eclampsia Community Guidelines (PRECOG).

Guidelines

Always use accurate equipment (a mercury sphygmomanometer or validated alternative method).

Use a sitting or semi-reclining position so that the arm to be used is at her heart level.

Do not take the blood pressure in the upper arm with the woman on her side, as this will give false, lower readings.

Use an appropriate cuff size:
• Standard size (13 x 23cm) for an arm circumference of up to 33cm.
• Large size (33 x 15 cm) for an arm circumference between 33 and 41cm.
• A thigh cuff (18 x 36cm) for an arm circumference of 41cm or more.

You will introduce less error by using too large a cuff than by using too small a cuff.

Deflate the cuff slowly, at a rate of 2 mmHg to 3 mmHg per second, taking at least 30 seconds to complete the whole deflation.

Use Korotkoff V (disappearance of heart sounds) for measurement of diastolic pressure, as this is subject to less intra-observer and inter-observer variation than Korotkoff IV (muffling of heart sounds) and seems to correlate best with intra-arterial pressure in pregnancy. In the case of the 15% of pregnant women whose diastolic pressure falls to zero before the last sound is heard, then both phase IV and phase V readings should be recorded (eg 148/84/0 mmHg).

Measure to the nearest 2 mmHg to avoid digit preference.

Obtain an estimated systolic pressure by palpation, to avoid an auscultatory gap.

If two readings are necessary, use the average of the readings and not just the lowest reading. This will minimize threshold avoidance (the tendency to repeat a reading until one that is below a known threshold is recorded that requires no action).

Definitions

Digit preference, whereby observers choose to record a favourite number, most commonly 0 or 5 mm Hg, is a serious source of bias. It is important to realise that such digit preference may introduce substantial errors that could lead to incorrect decisions being made, especially in patients with borderline blood pressures. Such bias is best avoided by recording systolic and diastolic pressures to the nearest 2 mmHg (Source: British Hypertension Society.)

The silent or auscultatory gap occurs when the sounds disappear between the systolic and diastolic pressures. The importance of the gap is that unless the systolic pressure is palpated first, it may be underestimated. The presence of a silent gap should be recorded on the case sheet or blood pressure chart. (Source: British Hypertension Society.)

Improving the reliability of proteinuria estimate using dipstick testing

These guidelines were prepared by Dr Jason Waugh.

The performance of a semi-quantitative dipstick is dependent on many variables, including how the dipstick is read (by all comers to a clinic; staff at a routine clinic; trained research observers; or a machine) and the urine concentration of the sample. The performance of quantitative methods of measuring protein is also dependent on a number of factors, such as the adequate collection of a 24 hour sample and the method used to measure protein.

Reduce false positive results by training the reader of the dipstick to use the correct methodology to read the dipstick tests. Manufacturers’ recommendations should be followed.

Automated dipstick readers reduce reader errors.

Do not repeat a test on a second sample as this does not improve the predictive value of a result for significant proteinuria.

Use a 24 hour urine collection to quantify excreted protein. The use of a protein/creatinine ratio instead of a 24 hour urinary protein requires local confirmation of performance, as the method of measuring proteinuria has been shown to modify the results.

Reduce concentration-related errors by assessing specific gravity or urine creatinine simultaneously with the protein dip result.

When required, confirm a 1+ result from a dipstick test for proteinuria by measuring protein excretion in a 24 hour urine collection