Understanding Your Kidney Function Test (KFT)

A kidney function test checks waste products in your blood to see how well your kidneys filter them [KDIGO, 2012]. It typically measures markers like creatinine, urea, eGFR, and uric acid [NICE, 2021].

What do creatinine and urea measure?

Creatinine is a normal waste product from everyday muscle breakdown [KDIGO, 2012]. Healthy kidneys filter creatinine out of your blood and pass it into urine [KDIGO, 2012].

Urea is a waste product formed in the liver when your body breaks down protein [NICE, 2021]. Both markers show how effectively the kidneys clear waste from circulation [KDIGO, 2012].

How is eGFR calculated?

Estimated glomerular filtration rate, or eGFR, reflects how many milliliters of blood your kidneys clean each minute [KDIGO, 2012].

Laboratories calculate eGFR using mathematical equations based on your blood creatinine, age, and sex [NICE, 2021]. An eGFR value of 90 or higher is generally considered normal kidney filtration [KDIGO, 2012].

What does uric acid show on a test?

Uric acid is a waste chemical created when the body digests purines found in certain foods and cells [NICE, 2021]. The kidneys remove most uric acid from your blood through urine [NICE, 2021].

Elevated uric acid levels can occur when kidneys filter less waste or when purine intake is high [KDIGO, 2012].

What factors can change your test levels?

Dehydration concentrates waste in the blood and can temporarily raise creatinine and urea [KDIGO, 2012]. High-protein diets or heavy meat consumption can also raise urea and creatinine levels [NICE, 2021].

People with higher muscle mass naturally produce more daily creatinine without having reduced kidney clearance [KDIGO, 2012].

Why is the printed lab range important?

Reference intervals vary because clinical laboratories use different testing instruments and chemical methods [CLSI, C28-A3]. Your report's printed reference range reflects the specific calibration of that analyzing machine [CLSI, C28-A3].

Comparing your results against the printed range provides the most accurate context for your numbers [CLSI, C28-A3].

Frequently asked questions

Does drinking less water change creatinine levels?

Yes, dehydration can cause creatinine levels to rise temporarily in blood tests [KDIGO, 2012].

Can intense exercise affect kidney test results?

Heavy physical exercise can break down muscle tissue and raise blood creatinine temporarily [KDIGO, 2012].

What does an eGFR value below 60 mean?

An eGFR consistently below 60 for three months indicates decreased kidney filtration capacity [KDIGO, 2012].

Why does high dietary protein affect urea results?

Digesting large amounts of protein creates extra urea that the liver releases into blood [NICE, 2021].

Why do creatinine ranges differ by biological sex?

Males typically have greater average muscle mass, which naturally produces higher baseline creatinine [KDIGO, 2012].

Sources

• Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Inter., Suppl. 2013; 3: 1–150.
• National Institute for Health and Care Excellence (NICE). Chronic kidney disease: assessment and management (NICE Guideline NG203), 2021.
• Clinical and Laboratory Standards Institute (CLSI). Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory; Approved Guideline—Third Edition. CLSI document EP28-A3c, 2010.