What Blood Test Tests the Kidneys?

The primary blood test that tests the kidneys is the serum creatinine test, which is used to calculate the estimated glomerular filtration rate (eGFR). This simple blood draw provides a reliable snapshot of how well your kidneys are filtering waste from your blood. Today, eGFR remains the gold standard for assessing kidney function, often combined with other markers like blood urea nitrogen (BUN) and cystatin C for a more complete picture. Understanding these tests is crucial for detecting early kidney disease, monitoring chronic conditions, and guiding treatment decisions, especially in the U.S. where kidney disease affects over 37 million adults.

Understanding Kidney Function Tests

Serum Creatinine and eGFR

Creatinine is a waste product from muscle metabolism that healthy kidneys filter out efficiently. When kidney function declines, creatinine levels rise. The eGFR is calculated using your creatinine level, age, sex, and race (though many labs now use a race-free equation). Today, the CKD-EPI 2021 equation is widely adopted, providing more accurate staging of chronic kidney disease (CKD). Normal eGFR is typically above 90 mL/min/1.73m², but values decline with age and other factors.

Blood Urea Nitrogen (BUN)

BUN measures the amount of nitrogen in your blood from the waste product urea. While not as precise as eGFR, BUN is often used alongside creatinine to evaluate kidney function and hydration status. A high BUN-to-creatinine ratio may indicate dehydration or other issues, while low values can signal liver problems. Normal BUN ranges from 7 to 20 mg/dL, but this can vary with diet and muscle mass.

Cystatin C

Cystatin C is a newer, more sensitive marker that is less influenced by muscle mass and diet. It is particularly useful when creatinine-based eGFR is unreliable (e.g., in elderly, malnourished, or very muscular individuals). Today, many labs offer cystatin C as a confirmatory test, and some guidelines recommend it for initial CKD screening in high-risk populations.

Why These Tests Matter today

With the rise of chronic diseases like diabetes and hypertension, kidney function testing has never been more critical. The U.S. Preventive Services Task Force continues to recommend routine screening for adults with risk factors. Early detection using these blood tests can slow CKD progression, reduce dialysis needs, and save lives. Newer innovations, such as point-of-care eGFR tests and home monitoring kits, are making kidney health management more accessible.

When to Get a Kidney Blood Test

If you experience any of the following, consider ordering a kidney function test:

• Swelling in legs, ankles, or feet (edema)
• Fatigue, weakness, or trouble concentrating
• Changes in urination frequency or appearance
• High blood pressure or diabetes
• Family history of kidney disease
• Age over 60

Normal Ranges for Kidney Function Tests

Understanding your results is key. Even if your numbers are within range, trends over time matter. If you have risk factors, ask your healthcare provider about the best kidney blood test for you.

Order Your Kidney Function Test Today

At anylabtests.com, you can order a comprehensive kidney function panel that includes eGFR, creatinine, BUN, and cystatin C. No doctor visit required—just a simple blood draw at a local lab. Take control of your kidney health today. Order your kidney test now and get results in days.