Protein in the urine always comes from the kidney and in general it implies significant kidney disease. Any renal disease may cause proteinuria. Glomerular disease may cause heavy proteinuria, but in many patients it is initially detected at lower levels. The level of proteinuria is a prognostic factor for most diseases – the higher, the poorer the renal prognosis.
Proteinuria may be caused by almost any renal lesion, but higher levels (>2g/d) are always caused by glomerular disease; some key causes are listed at nephrotic syndrome.
Usually asymptomatic, identified on dipstick test of urine
Protein/creatinine ratio, or 24h urine collection, to quantitate.
Proteinuria is probably not important if it:
- Only occurs following strenuous exercise
- Only occurs during a fever
- Only occurs during a UTI
- Is absent in the morning but occurs later in the day (orthostatic proteinuria)
Quantification:Ratios of protein or albumin to Creatinine (PCR, ACR) have largely replaced 24h collections for quantitating proteinuria. Albumin is about 70% of glomerular proteinuria at levels >1g/d. So very approximately:
Proteinuria 1g/day =PCR 100mg/mmol = ACR 70mg/mmol
- Proteinuria >100mg/mmol creatinine? (lower limit in young, who face a longer period at risk)
- Haematuria also present?
- Raised serum creatinine? (urgent if function deteriorating)
- Hypertension? (less suggestive with increasing age)
- Previous or family history suggest significant renal disease?
- Quantitate proteinuria and get previous creatinine values
- Ultrasound scan of kidneys may be valuable
- Consider referral (renal biopsy may be justified)
Proteinuria and cardiovascular risk
Even low levels of proteinuria (including microalbuminuria) are associated with increased mortality, mainly from cardoiovascular causes. The risk increases with the amount of proteinuria. The explanation for this is not known.
Management of low level proteinuria
In the absence of haematuria, hypertension or impaired renal function, or other symptoms, history or abnormalities, it is usually reasonable to monitor urine tests, blood pressure and renal function at 6 months, extending the interval to annually, indefinitely.