What laboratory tests are recommended for a 16-year-old boy with nocturia (nighttime peeing)?

Laboratory Tests for a 16-Year-Old Boy with Nocturia

The essential laboratory tests for a 16-year-old boy with nocturia should include urinalysis, renal function tests, thyroid function tests, calcium levels, and HbA1c to identify underlying medical causes that may be contributing to nighttime urination. 1

Initial Laboratory Evaluation

• Urinalysis: This is the minimum requirement to exclude urinary tract infection and hematuria 1

  • If urinalysis shows evidence of hematuria not associated with infection, urologic referral is indicated
  • If urinalysis is unreliable, consider urine culture

• Blood tests: 1

  • Electrolytes and renal function (BUN, creatinine)
  • Thyroid function tests
  • Calcium levels
  • HbA1c (to screen for diabetes)

• Urine albumin:creatinine ratio: To detect early kidney disease 1

Additional Testing Based on Clinical Suspicion

If sleep disorder is suspected:

• Ferritin level (especially if restless legs syndrome is suspected; supplementation if below 75 ng/ml) 1
• Consider referral for overnight oximetry if obstructive sleep apnea is suspected 1, 2

If cardiovascular disease is suspected:

• Electrocardiogram
• Brain natriuretic peptide (if heart failure is suspected) 1

If endocrine disorder is suspected:

• Morning urine osmolarity test after overnight fluid avoidance (for patients urinating >2.5L per 24h despite attempts to reduce fluid intake) 1
• If hypercalcemia is detected, parathyroid hormone levels 1

Diagnostic Tools to Supplement Laboratory Testing

• Frequency-volume chart (bladder diary): Have the patient record:
  • Time and volume of each void
  • Timing of fluid intake
  • Time of going to bed and waking up
  • Episodes of incontinence 1, 3

This helps distinguish between nocturnal polyuria (>35% of 24-hour urine output occurring at night) and reduced nocturnal bladder capacity 4

• Post-void residual assessment: Consider if there are obstructive symptoms or neurological concerns 1

Clinical Considerations and Pitfalls

• Nocturia in adolescents is often multifactorial - 76.5% of patients with nocturia may have nocturnal polyuria 5

• Sleep disorders are frequently overlooked as a cause of nocturia - studies show that 79.3% of awakenings attributed to nocturia may actually be caused by sleep disorders, particularly sleep apnea 2

• Evening fluid intake significantly correlates with nocturnal urine volume (r = 0.419, P = 0.002) 5

• Be aware that patients are extremely poor judges of the reasons they awake from sleep - they may attribute awakenings to bladder fullness when the primary cause is a sleep disorder 2

• Nocturia can have serious consequences including falls, fractures, reduced quality of life, and depression if left untreated 6

By systematically evaluating for these potential causes with appropriate laboratory testing, you can identify the underlying etiology of nocturia in this adolescent patient and direct treatment accordingly.