Creatinine and creatinine clearance blood tests
Creatinine and creatinine clearance blood tests may be arranged to help test how your kidneys are working, also called renal function.
Your kidneys do a vital filtration job. Every day, hundreds of times a day, all the blood in the body flows through the kidneys including through tiny filters called nephrons. Any waste products, including creatinine, are excreted in urine.
Creatinine and creatinine clearance
Creatinine is a waste product from the normal breakdown of muscle tissue. The kidneys filter creatinine from the blood into the urine.
The amount of blood the kidneys can make creatinine-free each minute is called the creatinine clearance rate. As kidney function declines, creatinine clearance also goes down.
In general, the creatinine clearance rate is a good estimate of the glomerular filtration rate.
Glomerular filtration rate (GFR)
The rate of blood flowing through the kidneys is the glomerular filtration rate, or GFR. The glomeruli are microscopic bundles of blood vessels inside nephrons, and are important parts of the filtering system.
The GFR can't be measured directly - which is where measuring creatinine and creatinine clearance comes in.
There are two main ways doctors test for creatinine levels:
- Creatinine clearance can be determined by measuring the amount of creatinine present in a urine sample collected in a special plastic container over 24 hours. This method isn’t very convenient but may be necessary to diagnose some kidney conditions.
- GFR can be estimated using a single blood test for levels of creatinine. This test is used more often than the 24-hour urine test. Different formulas are then used to take into account age, sex and sometimes weight and ethnicity. The higher the blood creatinine level, the lower the estimated GFR (eGFR) and creatinine clearance.
Understanding an abnormal creatinine test result
A low eGFR or creatinine clearance demonstrates kidney disease.
Repeated eGFR or creatinine clearance measurements over time can identify whether the kidney disease is acute (sudden, often reversible) or chronic (long-term and irreversible). Chronic kidney disease is also called CKD. More than 3 million people in the UK face chronic kidney disease.
Kidney function naturally declines with age but most people can lose more than half their renal function without having symptoms or any significant problems.
To determine the severity of chronic kidney disease doctors use a system that uses eGFR:
Stage 1: (G1) eGFR 90 or greater (normal kidney function although other tests may have detected kidney damage)
Stage 2: (G2) eGFR 60-89 (mild decline in kidney function)
Stage 3: (G3a and G3b) eGFR 30-59 (moderate decline in kidney function); this stage is divided into stage 3a (45-59) and stage 3b (30-44)
Stage 4: (G4) eGFR 15-29 (severe decline in kidney function)
Stage 5: (G5) eGFR less than 15 (kidney failure, usually requiring dialysis)
People over 60 may have a normal creatinine blood level, but still have a low eGFR and creatinine clearance. In that case the 24-hour urine collection test, or one of the eGFR formulas, may be able to more accurately identify the decline in kidney function.