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). It is freely filtered by the glomeruli and, under normal conditions, is not reabsorbed by the tubules to any appreciable extent. A small but significant amount is also actively secreted. Thus, creatinine production is proportional to muscle mass and varies little from day to day.
Serum creatinine varies with the subject’s age, body weight, and sex. It is sometimes low in subjects with relatively small muscle mass, cachectic patients, amputees, and in older persons. A serum creatinine level that would usually be considered normal does not rule out the presence of impaired renal function.
The assay of creatinine in serum or plasma is the most commonly used test to assess renal function. Measurements of creatinine are used in the diagnosis and treatment of renal disease and prove useful in the evaluation of kidney glomerular function and in monitoring renal dialysis. However, the serum level is not sensitive to early renal damage and responds more slowly than blood urea nitrogen (BUN) to haemodialysis during treatment of renal failure.Both serum creatinine and BUN are used to differentiate prerenal and postrenal (obstructive) azotemia. An increase in serum BUN without concomitant increase of serum creatinine is key to identifying prerenal azotemia. In post renal conditions where obstruction to the flow of urine is present e.g. malignancy, nephrolithiasis and prostatism, both the plasma creatinine and urea levels will be increased; in these situations the rise is disproportionately greater for BUN dueto the increased back diffusion of urea.
Chronic kidney disease is a worldwide problem that carries a substantial risk for cardiovascular morbidity and death. Current guidelines define chronic kidney disease as kidney damage or glomerular filtration rate (GFR) less than 60 mL/min per 1.73 m 2 for three months or more, regardless of cause.
Since a rise in blood creatinine is observed only with marked damage of the nephrons, it is not suited to detect early stage kidney disease. A considerably more sensitive test and better estimation of glomerular filtration rate (GFR) is given by the creatinine clearance test based on creatinine’s concentration in urine and serum or plasma, and urine flow rate. For this test a precisely timed urine collection (usually 24 hours) and a blood sample are needed. However, since this test is prone to error due to the inconvenient collection of timed urine, mathematical attempts to estimate GFR based only on the creatinine concentration in serum or plasma have been made. Among the various approaches suggested, two have found wide recognition: that of Cockroft and Gault and that based on the results of the MDRD trial. While the first equation was derived from data obtained with the conventional Jaffé method, a newer version of the second is usable for IDMS-traceable creatinine methods. Both are applicable for adults. In children, the Bedside Schwartz formula should be used.
In addition to the diagnosis and treatment of renal disease, the monitoring of renal dialysis, creatinine measurements are used for the calculation of the fractional excretion of other urine analytes (e. g., albumin, α‐amylase).
Creatinine Jaffé - Compensated Method
Serum and plasma
| Adults | ||
| Females | 44-80 μmol/L | 0.50-0.90 mg/dL |
| Males | 62-106 μmol/L | 0.70-1.20 mg/dL |
| Children | ||
| Neonates premature | 25-91 μmol/L | 0.29-1.04 mg/dL |
| Neonates full term | 21-75 μmol/L | 0.24-0.85 mg/dL |
| 2-12 m | 15-37 μmol/L | 0.17-0.42 mg/dL |
| 1-< 3 y | 21-36 μmol/L | 0.24-0.41 mg/dL |
| 3-< 5 y | 27-42 μmol/L | 0.31-0.47 mg/dL |
| 5-< 7 y | 28-52 μmol/L | 0.32-0.59 mg/dL |
| 7-< 9 y | 35-53 μmol/L | 0.40-0.60 mg/dL |
| 9-< 11 y | 34-65 μmol/L | 0.39-0.73 mg/dL |
| 11-< 13 y | 46-70 μmol/L | 0.53-0.79 mg/dL |
| 13-< 15 y | 50-77 μmol/L | 0.57-0.87 mg/dL |
Urine
| 1st morning urine | ||
| Females | 2.47‐19.2 mmol/L | 28‐217 mg/dL |
| Males | 3.45‐22.9 mmol/L | 39‐259 mg/dL |
| 24 h urine | ||
| Females | 7‐14 mmol/24 h | 740‐1570 mg/24 h |
| Males | 9‐21 mmol/24 h | 1040‐2350 mg/24 h |