This information is from guidelines issued by the National Institute of Clinical Excellence (NICE), UK in September 2008.
Using a single early morning sample of urine, Microalbuminuria is defined as a urine albumin:creatinine ratio of more than 2.5mg per mmol of creatinine in men ( more than 3.5mg per mmol of creatinine in women) but less than 30mg per mmol of creatinine.
Using a 24-hour collection of urine, Microalbuminuria is defined as an albumin concentration of more than 20mg/L but less than 300mg/L in 24 hour urine.
Proteinuria means that albumin or protein is readily detectable in the urine. This happens when the urine albumin: creatinine ratio is more than 30mg per mmol of creatinine. In such cases the 24 hour urine albumin excretion will be more than 300mg/L in 24 hours.
The protein in the urine is not entirely albumin. Globulins are also present. So, urine albumin is not the same as urine protein. Urine albumin comprises slightly more than half of the protein in the urine. As a rule, 30mg or urine albumin is equal to 50mg of urine protein. Therefore, proteinuria means the urine contains 500mg/L of protein in a 24 hour collection.