Urinary protein excretion greater than 150 mg/day that persists beyond a single measurement should be evaluated. Proteinuria may be benign or suggestive of glomerular disease. Heavy proteinuria (>3 g/day), lipiduria, and edema are indicative of glomerular disease.There are three types of proteinuria: glomerular, tubular, and overflow proteinuria. Glomerular proteinuria accounts for virtually all cases of persistent proteinuria and is the only kind that is identified by urine dipstick. Qualitative tests for proteinuria include urine dipsticks and the sulfosalicylic acid test. These tests provide only rough estimates of the degree of proteinuria since they are influenced by the urine volume. The standard urine dipstick detects only albumin and is not sensitive enough to detect microalbuminuria. Sulfosalicylic acid (SSA) detects all proteins in the urine. Quantitative determination of the degree of proteinuria is provided by a 24-hour urine measurement. Such testing is cumbersome and an alternative, particularly for serial monitoring, is estimation of the total protein-to-creatinine ratio, which correlates with daily protein excretion. The optimal approach to the patient with proteinuria includes a thorough history, physical examination, and urinalysis. Transient and orthostatic proteinuria should be excluded. Persistent proteinuria warrants a thorough evaluation even when accompanied by a normal urine sediment. The evaluation should include measurement of serum creatinine and an ultrasound examination to rule out structural causes. Patients with persistent proteinuria should be referred to a nephrologist for decisions regarding further management including renal biopsy. A renal biopsy may be performed in the setting of nephrotic syndrome, increasing protein excretion, or an elevation in the plasma creatinine concentration. The renal prognosis of patients with glomerular proteinuria relates to the quantity of protein excreted. Non-nephrotic proteinuria (less than 3 g/day)
TYPES OF PROTEINURIA —
There are three basic types of proteinuria — glomerular, tubular, and overflow . Only glomerular proteinuria (ie, albuminuria) is identified on a urine dipstick. Almost all cases of persistent proteinuria are due to glomerular proteinuria.
Glomerular proteinuria —
Glomerular proteinuria is due to increased filtration of macromolecules (such as albumin) across the glomerular capillary wall. The proteinuria associated with diabetic nephropathy and other glomerular diseases, as well as more benign causes such as orthostatic or exercise-induced proteinuria fall into this category. Most patients with benign causes of isolated proteinuria excrete less than 1 to 2 g/day.
Tubular proteinuria — Low molecular weight proteins — such as ß2-microglobulin, immunoglobulin light chains, retinol-binding protein, and amino acids — have a molecular weight that is generally under 25,000 in comparison to the 69,000 molecular weight of albumin. These smaller proteins can be filtered across the glomerulus and are then almost completely reabsorbed in the proximal tubule. Interference with proximal tubular reabsorption, due to a variety of tubulointerstitial diseases or even some primary glomerular diseases, can lead to increased excretion of these smaller proteins .
Tubular proteinuria is often not diagnosed clinically since the dipstick for protein does not detect proteins other than albumin and the quantity excreted is relatively small. The increased excretion of immunoglobulin light chains (or Bence Jones proteins) in tubular proteinuria is mild, polyclonal (both kappa and lambda), and not injurious to the kidney. This is in contrast to the monoclonal and potentially nephrotoxic nature of the light chains in the overflow proteinuria seen in multiple myeloma.
Overflow proteinuria — Increased excretion of low molecular weight proteins can occur with marked overproduction of a particular protein, leading to increased glomerular filtration and excretion. This is almost always due to immunoglobulin light chains in multiple myeloma, but may also be due to lysozyme (in acute myelomonocytic leukemia), myoglobin (in rhabdomyolysis), or hemoglobin (in intravascular hemolysis) [6]. In these settings, the filtered load is increased to a level that exceeds the normal proximal reabsorptive capacity. Patients with myeloma kidney also may develop a component of tubular proteinuria, since the excreted light chains may be toxic to the tubules, leading to diminished reabsorption.
Some patients have mixed forms of proteinuria. As an example, glomerular diseases such as focal segmental glomerulosclerosis can be associated with proximal tubular injury, leading to tubular proteinuria. In addition, patients with multiple myeloma and Bence Jones proteinuria can also develop nephrotic syndrome due to AL (primary) amyloidosis.
