Background: The value of nocturnal minute-by-minute urine was analyzed for the detection of microalbuminuria in high blood pressure.
Methods: Urinary albumin excretion (UAE) was measured by immunonephelometry in 70 patients with essential arterial hypertension (34 males, mean age 44 +/- 5 years, body mass index [BMI] 28 +/- 4, clinical blood pressure [BP] 157 +/- 12/97 +/- 7 mmHg) and 12 healthy normotensive controls (5 males, mean age 38 +/- 4 years, BMI 27 +/- 3; clinical BP 126 +/- 12/79 +/- 5 mmHg). Both 24 hours urine as well as nocturnal urine were collected over 2 days. The intraindividual variability was evaluated by calculation of the intraclass correlation coefficient and by the Bland and Altman method.
Results: Mean 24-hour UAE was 24.7 +/- 41.9 micrograms/min, greater than nocturnal urine (17.7 +/- 32.8 micrograms/min) (p < 0.05). The UAE rate was significantly greater in the hypertensive patients than in the controls (p < 0.01) for both the 24-hour and nocturnal urine. The intraindividual variability estimated as the percentage of repeatability was 35% for 24-hour UAE and 43% for nocturnal UAE, being slightly higher values than those observed in the controls. Microalbuminuria was detected in 17 (24%) of the patients in 24-hour samples while in the nocturnal urine 12.12 or 10 patients were found according to whether the threshold considered was 14 micrograms/min, 14.9 micrograms/min or 20 micrograms/min, respectively. While the specificity in nocturnal minute-by-minute urine was 98% and the positive predictive value was 0.91 for any of the thresholds considered, sensitivity ranged from 59% for the higher threshold (20 micrograms/min) to 70% for the lower threshold (14 micrograms/min).
Conclusions: The determination of microalbuminuria in minuted nocturnal urine in patients with essential arterial hypertension is less sensitive than that determined in 24-hour urine.