What lab tests should be ordered for a new onset of decreased urination?

Laboratory Evaluation for New Onset Decreased Urination

For a patient presenting with new onset decreased urination (oliguria), order a comprehensive metabolic panel (including BUN and creatinine), urinalysis with microscopy, and complete blood count with differential to evaluate for acute kidney injury, urinary tract obstruction, infection, or systemic causes. 1, 2

Initial Laboratory Workup

Essential First-Line Tests

• Urinalysis with dipstick and microscopy should be performed as the initial screening test, checking for leukocyte esterase, nitrite, protein, blood, and white blood cells 2, 3
• Basic metabolic panel including blood urea nitrogen (BUN) and serum creatinine is essential to assess renal function and identify acute kidney injury 1
• Complete blood count with differential should be obtained to evaluate for infection, with leukocytosis (WBC ≥14,000 cells/mm³) having high likelihood for bacterial infection 1, 4

Urinalysis Interpretation for Decreased Urination

• Pyuria (≥10 WBCs/high-power field) or positive leukocyte esterase/nitrite warrants urine culture with antimicrobial susceptibility testing 1, 2, 3
• Proteinuria (>2+ by dipstick) combined with red blood cell casts or deformed red blood cells suggests glomerular disease requiring further nephrology evaluation 1
• Negative dipstick for both leukocyte esterase AND nitrite effectively rules out UTI with 96% negative predictive value 3, 4

When to Add Blood and Urine Cultures

• If urosepsis is suspected (fever, shaking chills, hypotension, or delirium), obtain both urine and paired blood specimens for culture before initiating antibiotics 1, 2, 3
• Elevated band count (≥1,500 cells/mm³) has the highest likelihood ratio (14.5) for documented bacterial infection and warrants blood cultures 1

Proper Specimen Collection Technique

• For cooperative male patients: obtain midstream clean-catch specimen or use freshly applied clean condom catheter with frequent bag monitoring 1, 4
• For female patients: in-and-out catheterization is often necessary for proper specimen collection 1, 4
• For catheterized patients with suspected urosepsis: change the catheter prior to specimen collection and antibiotic initiation 1, 2

Additional Laboratory Considerations

• Spot urine calcium-to-creatinine ratio may be indicated if hypercalciuria is suspected as a cause of decreased urination 1
• Serum albumin should be checked if proteinuria is present, as levels <3.0-3.5 g/dL combined with significant proteinuria suggest nephrotic syndrome 1

Critical Pitfalls to Avoid

• Do NOT order urinalysis or urine culture for asymptomatic patients without specific urinary symptoms (dysuria, frequency, urgency, gross hematuria, new incontinence) or systemic signs 1, 3, 4
• Do NOT attribute decreased urination to UTI based solely on nonspecific symptoms like confusion, falls, or functional decline without specific urinary symptoms 3, 4
• Do NOT delay obtaining blood cultures if systemic infection is suspected, as leukocytosis (WBC ≥15,000 cells/mm³ or ≥20,000 cells/mm³) is associated with increased mortality in bloodstream infections 1

Algorithmic Approach

1. Assess for specific urinary symptoms (dysuria, frequency, urgency, hematuria, incontinence) or systemic signs (fever ≥100°F/37.8°C, hypotension, altered mental status) 3, 4
2. If symptoms present: order urinalysis with dipstick and microscopy, BUN/creatinine, and CBC with differential 2, 3, 4
3. If pyuria or positive leukocyte esterase/nitrite: proceed to urine culture 1, 2, 3
4. If systemic signs present: obtain paired blood and urine cultures before antibiotics 1, 2
5. If proteinuria or abnormal renal function: consider nephrology consultation for possible glomerular disease 1