Over collection or just a big guy

A patient came to my office with a creatinine of 2.2 indicating a GFR of 33mL/min by the MDRD formula. 

His primary care doctor ordered a 24 hour urine for creatinine and protein as part of her work-up for CKD:
  • 24-hour urine creatinine was 3,232 mg 
  • 24-hour urine protein was below the level of detection (<183>
To calculate the CrCl multiply the urine cr (total mass, not the concentration) by 100 then divide the product by 1440 (the number of minutes in 24-hours) and then by the serum creatinine (in mg/dl).
  • His CrCl is 102 mL/min
This is a huge discrepancy: 
  • Advanced Stage 3b CKD by MDRD
  • Normal kidney function by 24-hour urine collection
The first thing you should do is determine if the 24-hour urine was an adequate sample. Usually I worry about under-collections of urine due to a missed void or spillage. In this case I worried that an over-collection was masking renal failure.  (i.e. Did he collect his urine for more than 24-hours? Did his wife join in and contribute to the collection?) The average man produces 23 mg/kg of creatinine. The average woman produces 18 mg/kg. I am unaware of the proper figures for children.
His body weight is 123 kg and the 24-hour creatinine collection was 3,232 mg. This yields 23 mg/kg, right on the money for an average adult male.
This is just a big guy and this is where the MDRD can fail us.
Supporting the diagnosis of CKD stage zero was a normal renal ultrasound, a lock of proteinuria and a normal U/A and microscopic exam.