Creatinine (Jaffe and enzymatic methods)

Principle

• Jaffe reaction: creatinine reacts with picric acid in alkaline solution to form an orange-red complex, read at about 505–520 nm.
• Kinetic Jaffe measures the rate of colour formation in an early window (roughly 20–80 seconds), reducing interference from slow-reacting substances.
• Compensated Jaffe methods subtract a fixed value (about 0.3 mg/dL, 26 µmol/L) to correct for non-creatinine chromogens.
• Enzymatic method: creatininase converts creatinine to creatine; creatinase converts creatine to sarcosine and urea; sarcosine oxidase produces H2O2, which is measured by a peroxidase (Trinder) colour reaction.

Specimen

• Serum or heparin plasma; random or 24-hour urine (diluted) for clearance and ratios.

Results

• Typical adult serum creatinine: men about 0.7–1.3 mg/dL (62–115 µmol/L); women about 0.6–1.1 mg/dL (53–97 µmol/L). Local reference ranges apply.
• Conversion: mg/dL × 88.4 = µmol/L.
• eGFR is calculated from creatinine using equations such as CKD-EPI 2021.
• Creatinine clearance = (urine creatinine × urine volume per minute) / plasma creatinine.

Quality control and pitfalls

• Jaffe positive interference (falsely high): acetoacetate (ketoacidosis), pyruvate, glucose, ascorbic acid, some cephalosporins (e.g. cefoxitin).
• Bilirubin causes negative interference in Jaffe methods.
• Enzymatic methods are more specific and preferred for children and low values; dopamine and dobutamine can cause negative interference.
• Methods should be calibrated traceable to IDMS reference.
• Serum creatinine rises only after substantial loss of GFR and depends on muscle mass, diet (cooked meat) and age.

Clinical use

• Assessment of kidney function, staging of chronic kidney disease, detecting acute kidney injury, drug dosing, and urine albumin-to-creatinine ratio.