Sodium and blood pressure: positive associations in a north London population with consideration of the methodological problems of within-population surveys.

Blood pressure (BP) and 24-hr urinary sodium (Na) and potassium (K) excretion were measured in 58 men and women aged 40 and above who were selected randomly from the age-sex register of a general practice in North London. All 58 urine collections were reported as complete, but only 28/56 (50%) were classified as 'complete' by the excretion of a biological marker, p-aminobenzoic acid (PABA). Reliability of Na excretion estimated from repeated urine collections was 0.86, indicating that variability of Na excretion within individuals in this middle-aged and elderly population was low. Systolic blood pressure (SBP) was significantly related to 24-hr Na excretion and to 24-hr Na/creatinine ratio. After adjustment for age, sex and body mass index, a SBP-Na regression coefficient of 0.091 mmHg/mmol Na (P = 0.02) was observed. On simple regression analysis, a significant association was also found between diastolic blood pressure and Na (P = 0.04). In the sub-group classified as 'complete' collectors by PABA excretion, BP-Na regression coefficients were larger than in analyses of the sample as a whole. BP was not significantly related to K or Na/K.