Requirement: voiding diary recording, with each episode of noctural urination listed with volume in mL.
total noctural urine volume in mL =
= SUM(all nocturnal urine volumes)
nocturia index =
= (total noctural urine volume in mL) / (functional bladder capacity)
functional bladder capacity =
= MAX(single urine volume in mL)
NOTE: The nocturnal bladder capacity is based on the maximum urine volume at night. Normally this should be the same as the bladder capacity while awake.
Interpretation:
• A nocturia index <= 1 indicates no nocturnal urination or a single voiding.
• A nocturia index > 1 may arise either due to increased urine output (nocturnal polyuria) and/or low nocturnal bladder capacity.
• A large total nocturnal urine volume indicates nocturnal polyuria.
• A large number of voidings with small urine volumes indicates a low nocturnal bladder capacity.
Causes of nocturnal polyuria include:
(1) diabetes mellitus or diabetes insipidus
(2) mobilization of edema fluid in congestive heart failure
(3) disruption in secretion of arginine vasopressin
(4) sleep apnea or other respiratory disorders with raised concentrations of atrial natriuretic peptide
(5) excessive fluid ingestion before going to bed
Causes of a low nocturnal bladder capacity include:
(1) bladder obstruction (such as benign prostatic hypertrophy in males)
(2) bladder overactivity
(3) sensory urgency
(4) infection (cystitis)
(5) inflammation
(6) interstitial cystitis
(7) malignancy
Limitations:
• Since this is a ratio it suffers from the limits of ratios, such as cancelling, with small to small giving the same result as large to large.
