In nocturia evaluation, which assessment helps distinguish nocturnal polyuria from reduced bladder capacity?

• A. Bladder diary
• B. Urinalysis
• C. Sleep study
• D. Physical examination

Answer: (A)

Distinguishing nocturnal polyuria from reduced bladder capacity hinges on understanding urine production across day and night. A bladder diary records every void with time and volume over 24–48 hours, giving the total daily urine and the amount produced at night, plus how often you wake to void. From that, you can calculate how much of the total urine comes from nighttime (the nocturnal polyuria index). If nighttime urine is disproportionately high, nocturnal polyuria is the likely cause; if nighttime urine production isn’t elevated but there are frequent, small-volume nighttime voids, reduced bladder capacity is more likely. Urinalysis checks for infections or metabolic issues but doesn’t reveal patterns of production and storage. A sleep study looks at sleep stages and awakenings rather than urine flow. A physical exam doesn’t provide this urinary pattern information. So, keeping a bladder diary is the most informative approach to differentiate these two mechanisms.