Creatinine is the anhydride of creatine, which is found in muscle and inter-converted to phosphocreatine during muscle contraction. A proportion of muscle creatine undergoes spontaneous hydrolysis to creatinine, which is excreted.
A 70 kg man of normal muscle mass excretes 1 - 2 grams of creatinine per day: hence the practice of correcting urinary metabolites to a concentration "per gram of creatinine" to give an indication of the daily excretion rate.
This is of course an approximation: plasma creatinine increases after a meat meal rising, for example, from a mean of 90 umol/L to 175 umol/L three hours after a goulash containing 250 - 300 g of beef (data from Jacobsen et al, Lancet 1980; i: 319). An increased urine excretion rate follows.
Hence the comparison of urine results adjusted to the creatinine concentration assumes a constant diet and no significant change in muscle mass. However, intra-individual variation in urine creatinine is likely to be less than 10% per day.
The molecular weight of creatinine is 113.1. Hence 1 gram of creatinine in one litre of urine is of concentration 8.84 mmol/L.
To convert mmol/L to grams/L of urine creatinine, multiply the creatinine result by 0.1131.
Then correct the urine mercury (or other analyte) concentration to 1 gram of creatinine.
eg urine Hg 2 ug/L, creatinine 17.4 mmol/L, ratio 0.115
Creatinine = 17.4 x 0.1131 = 1.968 grams
So urine Hg = 1 ug/per gram of creatinine.
(Dr Nicholas Miller Feb. 2001)
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