A COMPARISON OF OVERNIGHT AND 24-HOUR COLLECTION TO MEASURE URINARY CATECHOLAMINES

The period of urine collection used to measure excretion of catecholam ines varies in epidemiological practice, We set out to compare overnig ht with 24 hour collection. Twenty-four subjects each collected urine for 24 hours, with the overnight urine being separately collected. The correlation of overnight and 24 hour catecholamines was highest when both measures were standardised for creatinine excretion and when crea tinine excretion was adjusted for urine flow rate. The observed correl ations were 0.74 for dopamine, 0.81 for noradrenaline and 0.54 for adr enaline. The use of overnight collections may therefore require a samp le size up to 1.5 times as large (for noradrenaline) or 3.4 times as l arge (for adrenaline) to achieve the same power as with 24 hour collec tions, However, the figures given exaggerate the advantage of 24 hour collections if these incorporate measurement errors that are not prese nt in overnight collections.