Validation of self-measurement of blood pressure in the control of hypertension in primary care: reference values and measurement days

The main objective of the study is to determine the clinical utility of 1, 2 and 3 days SMBP (not counting and counting the first day) in the detection of poor or good HBP control in Primary Care. This is a descriptive and cross-sectional study performed at the Health Care Centre of La Puebla de Montalbán (Toledo, Spain), in hypertensive patients of both sexes, 18-90 years old. The patients’ BP was measured consecutively at the clinic (CBP) with a mercury sphygomanometer, using SMBP (validated instrument) and ABPM (gold standard). The Canadian School of Clinical Epidemiology criteria were used to calculate the sample size. To evaluate HBP control were analysed in all SMBP regimens the BP values of 135/85, 137/87, 138/88, 137/88, 138/87, 130/80, 150/90, 140/85, 135/90, 140/90, 160/90, 155/90, 150/95, 160/95, 130/95, 125/90, 120/80 and 125/80 mm Hg.The validity indicators (VI)were calculated. The quality of the test (ROC curves) and the actual degree of agreement not resulting from chance (Kappa index). A total of 163 patients completed the study (mean age=65.6±12.5 years). Of these patients, 87.1% had poor HBP control with CBP,with 3 days SMBP not counting the first day (the best correlation with ABPM: r=0.66/0.71; SBP/DBP)=54.6%, ABPM=42.3%. VI to rule out poor HBP control (BP<135/85 mm Hg) and to confirm poor control (BP > or = 135 and/or 85 mm Hg): sensitivity=84.3%, specificity=66.6%. A remarkable reduction in false positives was observed on increasing the number of SMBP days. The best VI in the analysis of the remaining SMBP values were also obtained with 3 days not counting the first day, but establishing poor HBP control at > or = 140 and/or 90 mm Hg: sensitivity=78.4%, specificity=82.4%, +predictive value (PV)=76.9%, -PV=83.6%, +probability ratio (PR)=4.4, -PR=0.3, clinical utility level C (slight changes), area under the ROC curve=0.63 and Kappa index=0.61. The results of this study demonstrate that with the BP average <135/85 mm Hg, 3 days SMBP not counting the first day could replace ABPM in clinical practice for to be ruled out the poor HBP control, but when this average is > or = 135 and/or 85 mm Hg, SMBP is much more sensitive than specific (more false positives than false negatives), therefore ABPM should be used to confirm poor control. However, if poor HBP control by SMBP is established at an average > or = 140 and/or 90 mm Hg, the correlation between SMBP and ABPM is greatly increased, reducing the false positives.

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