Kidney Function Tests: eGFR, Creatinine, BUN & CKD Stages Explained
How to interpret your kidney markers — from the 2024 KDIGO guidelines to practical next steps
Your kidneys filter 200 liters of blood daily. Creatinine, BUN, and eGFR are the key markers tracked on routine blood work. The 2024 KDIGO guidelines updated CKD staging and added new treatment pathways with SGLT2 inhibitors. Here's how to read your kidney numbers.
Kidney function is assessed on every CMP and BMP, yet most patients have no idea what their creatinine or eGFR actually means until something goes wrong. Chronic kidney disease (CKD) affects approximately 1 in 7 US adults — about 37 million people — and most don't know they have it because early-stage CKD has no symptoms.
The 2024 KDIGO (Kidney Disease: Improving Global Outcomes) clinical practice guideline — the first major update since 2012 — reinforced eGFR as the primary metric for CKD classification and expanded treatment recommendations to include SGLT2 inhibitors and GLP-1 receptor agonists for kidney protection even in non-diabetic CKD. Here's what your kidney markers mean and when to be concerned.
Key Kidney Markers
| Marker | Normal Range | What It Measures |
|---|---|---|
| Creatinine | Men: 0.7 – 1.3 mg/dL / Women: 0.6 – 1.1 mg/dL | Waste product from muscle metabolism filtered by kidneys |
| BUN (Blood Urea Nitrogen) | 7 – 20 mg/dL | Protein metabolism byproduct filtered by kidneys |
| eGFR | >60 mL/min/1.73m² (normal >90) | Estimated glomerular filtration rate — how well kidneys filter |
| BUN/Creatinine Ratio | 10:1 – 20:1 | Distinguishes pre-renal (dehydration) from renal causes |
| Cystatin C | 0.56 – 0.99 mg/L | Alternative filtration marker less affected by muscle mass |
Important: Creatinine is influenced by muscle mass — a muscular 25-year-old male may have a creatinine of 1.3 mg/dL that is perfectly normal, while 1.3 in a petite elderly woman could indicate significant kidney impairment. This is why eGFR (which adjusts for age and sex) is more reliable than creatinine alone.
CKD Stages (2024 KDIGO Classification)
| Stage | eGFR (mL/min/1.73m²) | Description | Clinical Significance |
|---|---|---|---|
| G1 | ≥90 | Normal or high | CKD only if other markers abnormal (proteinuria, structural) |
| G2 | 60 – 89 | Mildly decreased | Common with aging; monitor annually |
| G3a | 45 – 59 | Mildly to moderately decreased | Referral to nephrology recommended; SGLT2i may be indicated |
| G3b | 30 – 44 | Moderately to severely decreased | Active management; dose-adjust medications |
| G4 | 15 – 29 | Severely decreased | Preparation for renal replacement therapy |
| G5 | <15 | Kidney failure | Dialysis or transplant consideration |
The KDIGO framework combines eGFR stage with albuminuria category (A1-A3) to create a risk heat map. High albuminuria (A3: >300 mg/g) at any eGFR stage significantly increases cardiovascular and kidney-failure risk. This is why a urine albumin-to-creatinine ratio (UACR) should accompany eGFR for complete kidney assessment.
When to Worry About Creatinine
Isolated creatinine elevation is very common and often benign. Before assuming kidney disease, consider:
- Dehydration: The #1 cause of a single borderline-high creatinine. BUN/creatinine ratio >20:1 supports this. Rehydrate and retest.
- High muscle mass: Bodybuilders and heavy athletes routinely have creatinine >1.3 with perfectly healthy kidneys.
- Recent high-protein meal: Eating a large amount of cooked meat 12–24 hours before a test can transiently raise creatinine.
- Medications: NSAIDs (ibuprofen, naproxen), ACE inhibitors, and ARBs can raise creatinine 10-20% — this is often expected and not dangerous.
- Actual concern: Creatinine that has risen >0.3 mg/dL from a recent baseline, eGFR consistently below 60, or presence of proteinuria (protein in urine). These warrant further evaluation.
Frequently Asked Questions
What is eGFR in a blood test?
eGFR (estimated glomerular filtration rate) measures how efficiently your kidneys filter waste from blood, expressed in mL/min per 1.73m² of body surface area. It's calculated from your creatinine, age, and sex using the CKD-EPI equation (updated 2021 to remove race adjustment). Normal eGFR is above 90; below 60 sustained for 3+ months meets the definition of chronic kidney disease. eGFR is more clinically useful than creatinine alone because it accounts for body size.
What is a dangerous creatinine level?
Creatinine above 4.0 mg/dL generally indicates severe kidney impairment (roughly Stage 4-5 CKD). A sudden rise of 0.3 mg/dL or more within 48 hours, or a doubling from baseline, suggests acute kidney injury and warrants urgent evaluation. However, context matters — a creatinine of 1.5 in a small elderly woman is much more concerning than 1.5 in a muscular young man. Always look at eGFR rather than creatinine alone.
What are the stages of chronic kidney disease?
CKD has five stages based on eGFR: Stage 1 (eGFR ≥90, kidney damage present), Stage 2 (60-89, mild decrease), Stage 3a (45-59, mild-to-moderate), Stage 3b (30-44, moderate-to-severe), Stage 4 (15-29, severe), and Stage 5 (<15, kidney failure). Stages 1-2 usually have no symptoms. Stage 3 is when monitoring and treatment intensify. The 2024 KDIGO guidelines recommend considering SGLT2 inhibitors starting at Stage 3 for kidney protection.
What does high BUN mean?
High BUN (blood urea nitrogen, above 20 mg/dL) most commonly indicates dehydration — check the BUN/creatinine ratio. If the ratio is >20:1, dehydration or GI bleeding is likely. If BUN and creatinine rise proportionally (ratio 10-20:1), kidney impairment is more likely. Other causes include high-protein diet, GI bleeding (digested blood becomes BUN), congestive heart failure, and catabolic states (burns, fever, surgery).
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