Which laboratory tests should be ordered to evaluate a patient with suspected nephrotic syndrome?

Laboratory Evaluation of Suspected Nephrotic Syndrome

Order a comprehensive initial laboratory panel including 24-hour urine protein quantification, serum albumin, complete metabolic panel, lipid profile, complete blood count, and targeted serologic studies to confirm nephrotic syndrome and identify secondary causes. 1, 2

Core Diagnostic Laboratory Tests

Urine Studies – Confirming Proteinuria

• 24-hour urine collection for total protein excretion is the gold standard; nephrotic-range proteinuria is defined as ≥3.5 g/24 hours in adults 1, 2, 3
• First morning spot urine protein-to-creatinine ratio (PCR) serves as an acceptable alternative when 24-hour collection is impractical; PCR >300–350 mg/mmol indicates nephrotic-range proteinuria 1, 3
• In children, use first morning urine PCR ≥2 g/g or proteinuria ≥40 mg/h/m² to define nephrotic-range proteinuria 2, 4
• Urinalysis with microscopy to assess for hematuria (>80% dysmorphic RBCs or red cell casts suggest glomerulonephritis rather than pure nephrotic syndrome) 1, 2

Serum Studies – Confirming Hypoalbuminemia and Assessing Function

• Serum albumin <3.0 g/dL defines hypoalbuminemia in adults; note that assay method matters—bromocresol green (BCG) reads approximately 0.5 g/dL higher than bromocresol purple (BCP) 1, 2, 4
• In children, serum albumin ≤2.5 g/dL is the traditional threshold 2, 4
• Serum creatinine and calculated eGFR to assess baseline kidney function 1, 2
• Complete metabolic panel including electrolytes, BUN, glucose, calcium, phosphate 1, 2
• Complete blood count with differential and platelets to detect anemia, infection, or hematologic malignancy 1, 2, 4

Lipid Assessment

• Fasting lipid panel including total cholesterol, LDL-C, HDL-C, triglycerides, apolipoprotein B, and lipoprotein(a) 2
• Hyperlipidemia is nearly universal in nephrotic syndrome and contributes to cardiovascular risk 2, 3

Secondary Cause Evaluation – Targeted Serologic Studies

Autoimmune Screening

• Antinuclear antibody (ANA), anti-double stranded DNA (anti-dsDNA), complement C3 and C4 to evaluate for systemic lupus erythematosus 1, 2, 4
• These tests should be ordered when clinical features suggest lupus or when complement-mediated glomerulonephritis is suspected 1, 2

Infectious Disease Screening

• Hepatitis B surface antigen (HBsAg) and hepatitis C antibody to identify viral-associated glomerular disease 1, 2
• HIV serology in high-risk populations or when HIV-associated nephropathy is suspected 2
• In congenital nephrotic syndrome (age <3 months), test for congenital syphilis and CMV 4

Paraprotein Screening (Adults Only)

• Serum and urine protein electrophoresis with immunofixation plus serum free light chains to exclude monoclonal immunoglobulin-associated renal disease in all adults with nephrotic syndrome 1, 2
• This is mandatory because paraprotein-related disease requires entirely different management 1

Disease-Specific Antibody Testing

• Anti-phospholipase A2 receptor (PLA2R) antibody when membranous nephropathy is suspected; positive serology may obviate the need for immediate biopsy in adults with typical presentation 1, 2, 4
• ANCA (anti-MPO and anti-PR3) if vasculitis is in the differential diagnosis 1, 2
• Anti-GBM antibody if rapidly progressive glomerulonephritis is suspected 2

Additional Laboratory Studies Based on Clinical Context

Thromboembolism Risk Assessment

• When serum albumin falls below 2.5–2.9 g/dL, assess thromboembolism risk factors including proteinuria >10 g/day, BMI >35 kg/m², heart failure, recent surgery, or prolonged immobilization 1, 2
• Membranous nephropathy carries particularly high venous thromboembolism risk 1, 2

Metabolic and Endocrine Evaluation

• Fasting glucose and HbA1c to identify diabetes mellitus, the most common secondary cause of adult nephrotic syndrome 2
• Thyroid function tests as hypothyroidism can accompany nephrotic syndrome 2
• Vitamin D, PTH, calcium, phosphate, alkaline phosphatase to assess bone-mineral metabolism 2

Genetic Testing Indications

• Consider genetic testing in children with congenital nephrotic syndrome (age <3 months), familial kidney disease, syndromic features, or steroid-resistant disease 1, 2, 4
• In adults with steroid-resistant FSGS, 11–24% will have disease-causing variants in podocyte or type IV collagen genes 1

Common Pitfalls to Avoid

Albumin Assay Variability

• Always document which albumin assay method your laboratory uses (BCG vs. BCP); a BCG result of 2.5 g/dL equals approximately 2.0 g/dL by BCP, which significantly affects thromboembolism risk stratification and treatment decisions 1, 2

Unnecessary Routine Serologies

• Do not order extensive autoimmune panels reflexively; routine serologic testing in the absence of clinical suspicion does not improve diagnostic accuracy and is rarely abnormal without suggestive history or physical findings 5
• Target serologic studies based on age, ethnicity, clinical presentation, and presence of extrarenal manifestations 1, 2

Timing of Laboratory Assessment

• Obtain baseline labs before initiating immunosuppressive therapy to avoid obscuring diagnostic findings and to establish accurate disease severity 1, 2
• In children <12 years with typical presentation, empiric glucocorticoid therapy without biopsy is standard; reserve extensive serologic workup for atypical features or steroid resistance 2, 4

Proteinuria Quantification Errors

• Random spot urine PCR has high day-to-day variability; first morning specimens are preferred, and 24-hour collections remain the gold standard when feasible 1, 2
• Transient elevations in proteinuria can occur with fever, exercise, urinary tract infection, or marked hyperglycemia; confirm persistent proteinuria before diagnosing nephrotic syndrome 2, 6