Creatinine

Creatinine is a breakdown product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass). 

Creatinine in blood is filtered by the kidneys for excretion in urine. If the filtering mechanism of the kidney is deficient, clearance of blood creatinine is affected, and blood creatinine levels rise. 

Doctors accept that high blood creatinine levels is an indicator of poor renal function in patients. However, this should be interpreted based on the patient's background and daily activities. Blood creatinine is not the sole marker of renal function. Muscular persons who do physical activities or workouts tend to have higher blood creatinine than less muscular persons who do activities and workouts.

There are lab tests, calculations and imaging investigations for determining renal function (function of the kidneys).

1. Blood creatinine
2. Creatinine clearance (calculated, need both blood & urine creatinine values)
3. Creatinine clearance (calculated based on age and blood creatinine)
4. Urea
5. Urea/Creatinine ratio
6. Cystatin C
7. Renal Function Tets (RFT) (routine panel of blood tests: sodium, potassium, chloride, phosphate, urea and creatinine)
8. Ultrasound of the kidneys, ureters and urinary bladder (KUB)
9. CT Scan 
10. CT Urography

Measuring serum creatinine is a simple test and it is the most commonly used indicator of renal function. 

It should be remembered that rise in blood creatinine levels is observed only with marked damage to functioning nephrons; therefore this test is not suitable for detecting early kidney disease. 

A better estimation of kidney function is given by the creatinine clearance test, which, however, is less convenient and unsuitable as a screening test because it requires the collection of urine over 24 hours (24-hour urine).

Reference range and interpretation

• The reference range for creatinine is 50 to 110 μmol/L (0.8 - 1.4 mg/dl). 
• Men tend to have higher levels of creatinine because they have more skeletal muscle than women. 
• Serum creatinine is not affected by diet or activity level.

External links

Creatinine

http://www.doctorslounge.com/nephrology/labs/creatinine.htm

Urea, Creatinine and Urea/Creatinine Ratio

https://acutecaretesting.org/en/articles/urea-and-creatinine-concentration-the-urea-creatinine-ratio