How do I interpret a Kidney Function Test (KFT) report?

How to Interpret a Kidney Function Test (KFT) Report

Interpreting a KFT report requires systematic evaluation of serum creatinine, estimated GFR (eGFR), and urine albumin/protein levels to classify kidney function into GFR categories (G1-G5) and albuminuria categories (A1-A3), which together determine disease severity and prognosis. 1

Core Components to Evaluate

1. Serum Creatinine and eGFR Assessment

• Use eGFR calculated from serum creatinine (eGFRcr) as your primary assessment tool, not serum creatinine alone 1, 2
• The 2009 CKD-EPI equation is the preferred calculation method for eGFR 2
• Serum creatinine alone is inadequate because it reflects not only kidney excretion but also creatinine generation, dietary intake, and metabolism 3

Important caveat: Serum creatinine can be misleading in patients with extremes of muscle mass, dietary protein intake, or certain medications that interfere with measurements 2

2. GFR Categories (G1-G5)

Classify kidney function based on eGFR values 2:

• G1: ≥90 mL/min/1.73m² (Normal or high GFR)
• G2: 60-89 mL/min/1.73m² (Mildly decreased GFR)
• G3a: 45-59 mL/min/1.73m² (Mildly to moderately decreased GFR)
• G3b: 30-44 mL/min/1.73m² (Moderately to severely decreased GFR)
• G4: 15-29 mL/min/1.73m² (Severely decreased GFR)
• G5: <15 mL/min/1.73m² (Kidney failure) 1

3. Albuminuria/Proteinuria Categories (A1-A3)

Evaluate urine protein using spot albumin-to-creatinine ratio (ACR) or protein-to-creatinine ratio (PCR) from first morning sample 2:

• A1: <30 mg/g (Normal to mildly increased)
• A2: 30-300 mg/g (Moderately increased)
• A3: >300 mg/g (Severely increased) 1, 2

Avoid outdated terms like "microalbuminuria" or "macroalbuminuria" - use the A1-A3 classification instead 1

4. Additional Laboratory Values

Evaluate these parameters to assess complications and guide management 2:

• Blood urea nitrogen (BUN): BUN-to-creatinine ratio helps differentiate prerenal (ratio >20:1), intrinsic renal (ratio 10-20:1), or postrenal causes
• Electrolytes: Sodium, potassium, chloride, bicarbonate, calcium, and phosphorus to identify metabolic complications
• Urinalysis with microscopy: Look for hematuria, pyuria, and cellular casts indicating active kidney damage 1

Interpretation Algorithm

Step 1: Determine if Kidney Disease is Present

Kidney disease requires BOTH:

• Decreased GFR (eGFR <60 mL/min/1.73m²) OR
• Evidence of kidney damage (albuminuria, abnormal urinalysis, structural abnormalities on imaging) 1

Step 2: Establish Duration

• Single abnormal result is insufficient - abnormalities must persist for >3 months to diagnose chronic kidney disease (CKD) 2
• If duration <3 months, consider acute kidney disease (AKD) or acute kidney injury (AKI) 1

Step 3: Classify Using CGA System

Use the combined CGA classification (Cause, GFR category, Albuminuria category) rather than GFR alone 1:

• Identify the Cause (diabetic kidney disease, hypertensive nephrosclerosis, glomerulonephritis, etc.)
• Assign GFR category (G1-G5)
• Assign Albuminuria category (A1-A3)

Example: "CKD G3b A2 due to diabetic kidney disease"

Step 4: Assess Risk and Prognosis

The combination of GFR and albuminuria categories determines risk for adverse outcomes (mortality, cardiovascular events, kidney failure, AKI progression) 1:

• Lower GFR + higher albuminuria = higher risk
• G1-G2 with A1 = lowest risk
• G4-G5 with A3 = highest risk

When to Use Confirmatory Tests

Consider cystatin C-based eGFR (eGFRcys) or combined creatinine-cystatin C eGFR (eGFRcr-cys) when 1:

• Extremes of muscle mass or body composition
• Dietary extremes (vegetarian, high protein intake)
• Medications interfering with creatinine secretion
• Clinical decisions require more accurate GFR assessment

Avoid cystatin C in patients with thyroid dysfunction - thyroid hormones significantly affect cystatin C levels independent of kidney function 4

Consider measured GFR using exogenous filtration markers (iothalamate, iohexol) when 1, 5:

• eGFR estimates are likely inaccurate
• Clinical decisions based on inaccurate estimates may have serious consequences (e.g., kidney donation evaluation, chemotherapy dosing)

Critical Pitfalls to Avoid

• Never diagnose CKD from a single test - require persistence >3 months 2
• Never use serum creatinine alone - always calculate eGFR 1, 3
• Never assess GFR without albuminuria - both are required for complete evaluation 1
• Never infer cause from comorbidity alone (e.g., assuming diabetic kidney disease just because patient has diabetes) - require supporting evidence 1
• Avoid outdated terminology like "renal insufficiency," "azotemia," "ESRD," or "chronic renal failure" 1

Special Clinical Contexts

Acute Changes in Kidney Function

When evaluating acute changes (AKI or AKD), perform minimal evaluation including 1:

• Detailed history: medications (including over-the-counter), recent illnesses, volume status, nephrotoxin exposures
• Physical examination: blood pressure (lying and standing), volume assessment
• Serum creatinine, eGFR, electrolytes, complete blood count
• Urinalysis with dipstick for proteinuria
• Point-of-care ultrasound to rule out obstruction

Monitoring Frequency

Tailor monitoring frequency to GFR category, albuminuria category, and rate of progression 2:

• Higher risk categories (lower GFR, higher albuminuria) require more frequent monitoring
• Rapidly declining kidney function warrants closer follow-up