Kidney Function Tests: A Simple Guide
Your kidneys are vital organs that act as your body's advanced filtration system. When doctors want to check how well these filters are working, they use a panel of simple blood tests.
These tests work by measuring specific waste products in your blood. When your kidneys are healthy, they filter these wastes out into your urine, keeping their levels in your blood low. When kidney function is impaired, these waste products build up, signaling a problem.
Serum Creatinine
This is the most common kidney function test. Creatinine is a waste product that comes from the normal wear and tear of muscles in your body. It's produced at a fairly constant rate.
- Test: A simple blood draw from a vein in your arm. Your doctor may ask you to avoid eating meat or taking creatine supplements for a day, as they can temporarily raise levels.
- Normal Range (Typical):
- Men: ~0.74–1.35 mg/dL
- Women: ~0.59–1.04 mg/dL
- Interpretation: A high serum creatinine level usually means your kidneys are not filtering waste properly.
Key Point: Creatinine is affected by muscle mass. A very muscular person will have a higher baseline creatinine than an elderly, frail person. This is why doctors almost always use it to calculate your eGFR.
Good eGFR
Poor eGFR
Estimated GFR (eGFR)
The eGFR is not a direct test but a calculation. It uses your serum creatinine level, age, and sex to *estimate* your Glomerular Filtration Rate—or how fast your kidneys are filtering blood. It's the best overall measure of kidney function.
- Normal eGFR: ≥90 mL/min/1.73 m²
- Chronic Kidney Disease (CKD): An eGFR below 60 for three months is the primary sign of CKD.
| CKD Stage | eGFR (mL/min/1.73 m²) | Kidney Function |
|---|---|---|
| Stage 1 | ≥90 | Normal (with other signs of damage) |
| Stage 2 | 60-89 | Mild Impairment |
| Stage 3 (a/b) | 30-59 | Moderate Impairment |
| Stage 4 | 15-29 | Severe Impairment |
| Stage 5 | <15 | Kidney Failure |
Blood Urea Nitrogen (BUN)
The BUN test measures urea, a waste product formed in the liver when protein is broken down. Like creatinine, healthy kidneys filter it out.
- Normal Range: 6–24 mg/dL (This can vary).
- High BUN: Can suggest kidney problems, but it's also easily affected by other factors.
Key Point: BUN is less specific than creatinine. A high BUN can also be caused by dehydration, a high-protein diet, or even bleeding in the gut. A very high BUN-to-Creatinine ratio (e.g., >20:1) often points to dehydration, not kidney disease.
Cystatin C
Cystatin C is a newer test. It measures a protein produced by all cells in your body at a constant rate. Because its production is not affected by muscle mass, age, or sex, it can be a more accurate marker than creatinine for some people.
- Interpretation: A high Cystatin C level indicates reduced kidney function (a low eGFR).
- Advantages: It's a great "confirmatory" test. It can detect kidney dysfunction earlier in people with very low or very high muscle mass, where creatinine might be misleading.
The Most Important Takeaway: These tests are almost always interpreted together. A single high BUN is not a concern, but a high BUN and a high Creatinine and a low eGFR all point to the same thing: the kidneys are not filtering properly.