Chronic Kidney Disease
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Proteinuria

 

Proteinuria is an important consequence of significant renal disease. The presence of proteinuria has important implications for chronic kidney disease. The most important of these is that proteinuric CKD is more likely to decline towards end stage renal failure, yet is more likely to respond to blood pressure control.

 

Measurement of proteinuria

 

This can be a confusing area. Several measurements can be used, and an understanding of their relative merits is important.

 

Urinalysis

This is quick an easy, giving a qualitative measure of proteinuria. The sticks used will not detect microalbuminuria. Usage of urinalysis is useful as an adjunct to an abnormal eGFR result, but it is not useful as a treatment end point.

Albumin/Creatinine ratio

Nephrologists rarely use this test. An ACR result falls into 3 bands

ACR result (mg/mmol) Implication
Male < 2.5 Female < 3.5 Normal
Male 2.5-30 Female 3.5-30 Microalbuminuria - refer to diabetes guidelines
More than 30 Overt proteinuria - use PCR instead

An early morning specimen is to be preferred but not mandatory.

ACR are useful in the management of diabetics, but once it is above 30, the patient has established diabetic nephropathy, and should be assessed by the CKD guidelines. Continued measurement of ACR is probably a waste of time.

24 hour urine collection for total protein

This remains the gold standard for the assessment of total protein urine excretion. However it is a cumbersome test that patients have difficulty completing. It has been superceded by protein/creatinine ratios. Therefore 24 hour collection should only be needed rarely.

Urine Protein/Creatinine ratio

This requires a small MSU bottle sample of urine, which can be untimed, although an early morning sample is preferable. The Derby laboratory returns a ratio that is dimensionless (mg protein/mg creatinine), but directly correlated with an estimate of total protein excretion - i.e. a result of 2.1 is equivalent to 2.1 G/day loss.

PCR result (mg/mg) Implication
< 0.20 Normal
0.20-0.50 Significant but satisfactory- continue monitoring
0.50-1.00 Significant - consider BP control and referral if CKD 3-5
1.00-3.00 Unsatisfactory - consider BP control and referral
3.00 or more Possible nephrotic range - referral indicated

Some laboratories will use different units - mg/mmol - divide by 100 to get the grams per day estimate (e.g. 1 gram excretion per day is equivalent to a PCR of 1.0 mg/mg or 100 mg/mmol).

If there are other markers of renal disease (e.g. an eGFR < 60 or microscopic haematuria) the threshold for referral should be lower.

Bence-Jones proteins

For completeness these are mentioned, but are not required as a part of the CKD referral process routinely. They are being replaced by the use of serum free light chain assays.

Summary

Advice can be summarised as dipstick urine. Then, if proteinuria is present, send a urine protein/creatinine ratio.