Proteinuria simply means the presence of protein in the urine. We all pass a small amount of protein in our urine – up to 150 mg daily. Proteinuria in excess of this may be classified as transient, postural or persistent. Transient proteinuria occurs after vigorous exercise, during febrile illnesses and in heart failure. Postural (aka orthostatic) proteinuria occurs in young adults after standing for prolonged periods. It disappears after lying down and can therefore be excluded in an early morning specimen. Postural proteinuria is completely benign. The rest of this chapter will be devoted to the discussion of persistent proteinuria, which may be further classified by severity into microalbuminuria, true proteinuria and nephrotic range proteinuria (see below).

There are three methods: the urine dipstick test, the 24 h urine collection and the measurement of albumin or protein:creatinine ratio in a spot specimen. Discussion of the advantage and limitations of these tests follows.

The dipstick test, which detects primarily albumin in the urine, is the simplest measure of proteinuria. It is a useful screening test but has a number of limitations as shown in the box:

Despite these reservations the urine dipstick test is an important and often neglected part of the initial assessment of patients with both AKI and CKD. It is surprising how often patients with AKI in particular are referred to nephrologists before their urine has been dipped for blood and protein.

Bence Jones proteins (BJP) are free light chains detected in the urine of patients with myeloma and primary amyloidosis. They are identified by electrophoresis rather than by the urine dipstick test which detects albumin and not globulins. Historically the investigation of a patient with suspected myeloma has been by blood for serum electrophoresis and urine for BJP, though this is changing with the introduction of highly sensitive serum free light chain assays. Clonal production of free light chains can be detected earlier in serum than urine because of the reabsorptive capacity of the renal tubules (up to 20–30 g of free light chains can be reabsorbed daily).

Up until recently the answer to ‘how do we quantify urine protein?’ would have been ‘by 24 h urine collection’. This method of measuring urine protein has recently been abandoned due to a combination of inaccuracies in collection, the cumbersome nature of this process and validation of a simpler test (see below). Patients are still asked to collect their urine for twenty four hours occasionally, for example when estimating residual renal function in a patient on peritoneal dialysis. To improve accuracy the following instructions are useful.