What initial laboratory tests should be obtained in a patient presenting with flank pain?

Initial Laboratory Tests for Flank Pain

Obtain urinalysis with microscopy, serum creatinine, complete blood count, and urine pregnancy test (in women of reproductive age) before proceeding to imaging. 1

Essential Laboratory Work-up

Urinalysis with Microscopy

• Urinalysis with microscopy is the cornerstone laboratory test, detecting hematuria, pyuria, bacteriuria, crystals, and casts that guide diagnosis and management. 1
• More than 20% of patients with confirmed urinary stones have negative urinalysis for hematuria, so absence of blood does not exclude stone disease. 1, 2
• The presence of even microscopic hematuria shifts probability toward stone disease and urinary tract obstruction. 1

Renal Function Assessment

• Serum creatinine must be obtained to establish baseline renal function before any imaging, particularly if contrast-enhanced studies are being considered. 1
• This is especially critical in patients with suspected bilateral obstruction or solitary kidney. 1

Infection Markers

• Complete blood count identifies leukocytosis (>10 × 10⁹/L), which is prominent in both infectious complications and renal infarction. 1, 3
• Urine culture should be obtained when infection is suspected, particularly if fever, dysuria, or pyuria are present. 1
• C-reactive protein may be measured if systemic inflammation or complicated infection is a concern. 1

Mandatory Pregnancy Testing

• Beta-hCG testing in all women of reproductive age must be performed before any imaging to rule out ectopic pregnancy, which can present with flank pain and delayed menses. 4, 1
• Do not delay this test—gynecologic emergencies such as ectopic pregnancy and ovarian torsion require urgent intervention. 1

Additional Laboratory Tests in Specific Clinical Scenarios

When Fever or Systemic Signs Are Present

• The combination of flank pain with fever creates diagnostic urgency for infectious etiologies like pyelonephritis or perinephric abscess. 5
• Fever, chills, or sepsis signs mandate immediate evaluation for obstructive pyelonephritis or urosepsis. 1, 5

When Renal Infarction Is Suspected

• Lactate dehydrogenase (LDH) levels >620 IU/L are prominent laboratory findings in renal infarction, which should be considered in patients with atrial fibrillation presenting with flank pain. 3
• Elevated LDH combined with leukocytosis in the absence of infection suggests vascular etiology. 3

Critical Clinical Pitfalls to Avoid

Do Not Assume Normal Urinalysis Excludes Pathology

• A normal urinalysis does not exclude significant urologic pathology—over 20% of confirmed stone patients may have negative findings. 1
• The clinical picture may have evolved, and hematuria can be intermittent or absent. 1

Do Not Miss Ectopic Pregnancy

• Failure to obtain pregnancy testing in women with delayed menses and flank pain can result in catastrophic missed diagnosis. 4, 1

Recognize Red Flags Requiring Urgent Evaluation

• Hemodynamic instability, shock, inability to urinate, or decreased urine output requires immediate imaging and possible admission regardless of laboratory results. 1, 5
• Signs of systemic infection (fever with hemodynamic changes) suggest complicated pyelonephritis or sepsis requiring emergent intervention. 5
• Solitary kidney with obstruction is a critical scenario demanding urgent evaluation. 1

Laboratory-to-Imaging Algorithm

After Laboratory Results

• If urinalysis shows hematuria without infection signs and renal function is normal, proceed to non-contrast CT abdomen/pelvis (98-100% sensitivity for stones). 1, 6
• If fever, pyuria, or leukocytosis are present with flank pain, contrast-enhanced CT may be more appropriate to evaluate for pyelonephritis or perinephric abscess. 5, 6
• If pregnancy test is positive, ultrasound becomes first-line imaging to avoid radiation. 4, 1
• If severe contrast allergy or significant renal insufficiency exists, ultrasound is preferred over CT. 1