What is the most appropriate next investigation for a patient presenting with nocturnal enuresis, weight loss, and polydipsia?

Most Appropriate Next Investigation

The most appropriate next investigation is an immediate urine dipstick test to detect glycosuria and proteinuria, followed by blood glucose measurement if glycosuria is present. 1, 2

Clinical Rationale

The triad of bedwetting (nocturnal enuresis), weight loss, and polydipsia is a medical red flag that demands urgent evaluation for diabetes mellitus or renal disease before considering any primary bladder disorder. 1, 2 This combination of symptoms represents classic hyperglycemia manifestations and should never be dismissed as simple primary enuresis. 1, 2

Diagnostic Algorithm

Step 1: Immediate Urine Dipstick Testing

• Perform urine dipstick testing immediately to detect glycosuria (indicating diabetes mellitus) and proteinuria (suggesting kidney disease). 1
• The presence of glycosuria on dipstick mandates immediate exclusion of diabetes mellitus. 1
• If proteinuria is detected, repeat testing should be performed, and if persistent, investigations for kidney disease should be initiated. 1

Step 2: Blood Glucose Testing Based on Dipstick Results

• If glycosuria is present: Proceed immediately with blood glucose testing (random plasma glucose or fasting plasma glucose). 1, 2
• A random plasma glucose ≥200 mg/dL in the presence of classic symptoms (polyuria/polydipsia and unexplained weight loss) is sufficient to diagnose diabetes mellitus without requiring additional testing. 2, 3
• If random glucose is elevated but <200 mg/dL, confirm with fasting plasma glucose or HbA1c. 2

Step 3: Additional Baseline Testing

• Obtain HbA1c, electrolytes, and renal function tests to assess disease severity and screen for kidney disease. 1
• Consider a frequency-volume chart for 2-3 days to document polyuria and quantify polydipsia. 1

Critical Pitfalls to Avoid

• Do not delay urine and glucose testing as this can lead to delayed diagnosis and progression to diabetic ketoacidosis. 2
• Do not order renal ultrasound or other expensive imaging before basic screening tests, as this wastes resources and delays diagnosis. 1, 2
• Do not dismiss this presentation as simple primary enuresis or psychogenic polydipsia without first ruling out systemic disease. 1, 2
• Do not perform routine renal ultrasound unless initial screening identifies specific risk factors such as history of UTI, hematuria, urolithiasis, or renal insufficiency. 1, 2

Key Historical Features to Document

• Quantify fluid intake using a bladder diary to distinguish pathologic polydipsia from normal intake. 1
• Assess frequency of daytime voiding to evaluate for polyuria versus bladder dysfunction. 1
• Document recent onset of fatigue or tiredness, which may suggest diabetes or kidney disease. 1
• Review current medications to ensure symptoms are not medication-related. 4

Why This Approach Takes Priority

The International Children's Continence Society and American Academy of Pediatrics explicitly state that weight loss combined with polydipsia and enuresis indicates potential diabetes mellitus that must be ruled out immediately. 1, 2 Urine dipstick testing is the most cost-effective, rapid, and appropriate initial screening test that guides all subsequent diagnostic steps. 1 This simple bedside test can be performed immediately and provides critical information that determines whether urgent diabetes evaluation or kidney disease workup is needed. 1