PROTEINURIA

While large amount of low molecular weight proteins, e.g. globulins and peptide hormones, are filtered through glomeruli, most of them are reabsorbed in proximal tubules. Normal protein excretion in urine does not exceed 100 mg/day or gt;4 mg/m²/hour.

Proteinuria may be detected semi-quantitatively by heat coagulation test, sulphosalicylic acid test or dipstick method; or quantitatively by Esbach's method (Ch 21.2). However, microalbuminuria (15-300 mg/day) precedes appearance of overt proteinuria, which can be detected by sensitive radioimmunoassay.

While 24-hour urinary protein excretion is more reliable indicator of the severity of proteinuria than random tests, it is often difficult to collect proper samples in children. Urinary protein: creatinine ratio (UPCR), with normal value of lt;0.2 mg/mg after 2 years of age in morning urine sample, correlates well with 24-hour urine proteins and is commonly used to quantify proteinuria in children.

Etiologically, proteinuria may be glomerular or tubular in origin (Table 21.8). Severity of proteinuria often correlates well with underlying pathology and may be divided as follows:

a. Transient mild proteinuria (lt;300 mg/day or UPCR lt;0.2 mg/kg)) is common in normal children during fever, stress or after heavy exercise and during non-renal illnesses, e.g. seizures, cardiac failure and does not reflect renal disease.

Orthostatic proteinuria is a benign condition with appearance of mild proteinuria in upright posture only, rarely exceeding 600 mg/m²/day and characterized by:

• No proteinuria in recumbent posture, i.e. in early morning sample,

• Significant proteinuria (+/++) in standing/ambulatory state, i.e.

• Absence of edema or other symptoms.

No treatment is required except follow-up and pro­gression to significant glomerular disease is rare.

b. Tubular proteinuria (lt;1 gm/1.73 m²/day or UPCR 0.2-2.0 mg/kg) is characteristically selective, i.e. containing low molecular weight plasma proteins with molecular weight less than that of albumin, e.g. peptide hormones or microglobulins. These proteins are normally filtered from glomeruli but reabsorbed in proximal tubules. Tubular proteinuria is seen in proximal tubular disorders, e.g. acute tubular necrosis, interstitial nephritis. renal tubular acidosis or hereditary nephropathies, usually associated with glucosuria, phosphaturia, bicarbonaturia and aminoaciduria. Edema is very rare as there is no albuminuria.

c. Nephrotic range proteinuria (gt;1 gm/m²/day or UPCR gt;2 mg/mg), may be selective with loss of plasma proteins having molecular weight less than or equal to that of albumin (minimal change nephrotic syndrome) or non-selective, also containing higher molecular weight proteins, e.g. IgG and transferrin. Severe edema is common. Minimal change nephrotic syndrome is the commonest cause of severe proteinuria in childhood.

21.7