Laboratory Workup for Kidney Pain
For a patient presenting with kidney pain, order urinalysis with microscopy, serum creatinine with eGFR calculation, complete blood count, and serum lactate dehydrogenase (LDH) as your initial laboratory panel. 1, 2
Essential Initial Laboratory Tests
Core Renal Function Assessment
- Serum creatinine with eGFR calculation using the CKD-EPI equation is the primary marker for kidney function assessment, as this provides more accurate staging than creatinine alone 1
- Blood urea nitrogen (BUN) should be measured alongside creatinine, as the BUN-to-creatinine ratio helps differentiate between prerenal, intrinsic renal, and postrenal causes of kidney dysfunction 1
- Urinalysis with microscopy is essential to detect hematuria, proteinuria, and cellular casts that may indicate the underlying cause of kidney pain 1, 3
Critical Diagnostic Markers
- Complete blood count (CBC) with white blood cell differential, as leukocytosis (typically >12,000/μL) is consistently elevated in acute renal pathology including renal infarction and pyelonephritis 2
- Serum lactate dehydrogenase (LDH) is particularly important, as elevated levels (often >1,500 IU/L) are found in all cases of acute renal infarction and help distinguish this from other causes of flank pain 2
Electrolyte and Metabolic Panel
- Comprehensive metabolic panel including sodium, potassium, chloride, calcium, phosphorus, and bicarbonate to evaluate for electrolyte imbalances and acid-base disturbances that may accompany acute kidney pathology 1
Urine Protein Quantification
- Spot urine albumin-to-creatinine ratio (ACR) or protein-to-creatinine ratio (PCR) from a first morning sample if proteinuria is detected on urinalysis, as this quantifies the degree of kidney damage 1
Clinical Context-Specific Testing
When Hematuria is Present
- Urine culture is indicated even with negative urinalysis dipstick, as lower bacterial levels may be clinically significant but not detectable on dipstick 3
- Hematuria detected on urine reagent strips in the setting of flank pain, leukocytosis, and elevated LDH strongly suggests acute renal infarction and warrants urgent imaging 2
When Renal Insufficiency is Detected
- Cystatin C can be used as a confirmatory test when eGFR based on serum creatinine may be less accurate due to extremes of muscle mass or other factors 1
Important Clinical Pitfalls
Acute Renal Infarction is Frequently Missed
The triad of unilateral flank pain + hematuria + markedly elevated LDH in a patient with thromboembolic risk factors (atrial fibrillation, hypercoagulable states) should immediately raise suspicion for acute renal infarction, which is often initially misdiagnosed as renal colic, pyelonephritis, or acute abdomen 2
Timing Considerations
- A single abnormal test result is insufficient for diagnosis of chronic kidney disease; persistence of abnormalities for >3 months is required to distinguish acute from chronic processes 1
- Certain medications and substances can interfere with creatinine measurements, affecting eGFR accuracy 1