Factors affecting creatinine clearance:
(1) sex: It is normally lower in women than men.
(2) age:
(2a) It is lower in children, until the age of 2.
(2b) It usually decreases in adults with age, starting at age 20.
(3) muscle mass
(4) pregnancy
(5) hyperglycemia: Due to osmotic diuresis and body fluid redistribution.
(6) patients with morbid obesity or marked ascites: They excrete less creatinine per kg than expected; you need to correct to lean body weight if these are marked.
(7) proteinuria: This results in increased creatinine clearance.
(8) time of day: It is highest in afternoon.
(9) race
(10) muscle pathology (myopathy, blunt trauma, ischemia)
(11) renal disease, with a decline in clearance as the disease becomes worse
Muscle mass is an important determinant of creatinine production. This contributes to the gender difference observed. It also explains the decrease seen in elderly associated with a decline in muscle mass. It can also be important in patients with cachexia.
The accuracy of creatinine clearance based on 24-hour urine collection is dependent on how well the patient follows instructions and upon preservation of the specimen.
The accuracy of the estimated creatinine clearance performed on a spot urine is affected by many variables such as time of day and activities prior to collection.
