Low-level proteinuria (<500 mg/m²/day) may occur in the setting of recent exercise, stress, fever, dehydration, and urinary tract infection. Transient proteinuria has been reported in up to 12% of AYAs.³ It is not associated with long-term renal morbidity.

Postural proteinuria is a condition in which low- to moderate-level proteinuria develops in the standing or upright position and disappears in the recumbent position. It is found in up to 20% of school-age children, occurring more commonly in the second decade of life. Postural proteinuria is slightly more common in males.² It often resolves by young adulthood and is rarely associated with long-term renal disease. The diagnosis of postural proteinuria is confirmed when a random urine specimen is positive for protein and a first morning urine specimen is negative to trace for proteinuria. Patients with postural proteinuria may have up to 1,000 mg of protein in a 24-hour sample. Although a benign condition, postural proteinuria can rarely become fixed proteinuria signaling the presence of true renal disease; therefore, AYAs with postural proteinuria should have first morning urinalyses checked at regular intervals (i.e., well-child visits) to evaluate for the presence of fixed proteinuria.

Persistent proteinuria that occurs in the absence of microscopic or gross hematuria may be due to glomerular and nonglomerular causes.

Hyperfiltration Renal Injury: An important risk factor for progressive CKD, hyperfiltration occurs with excessive filtration through remaining functioning nephrons to maintain the glomerular filtration rate (GFR). This mechanism is important in diabetic and reflux nephropathies and renal dysplasia.

Glomerulopathies: Isolated fixed proteinuria may be due to primary glomerular diseases. An important example of this is focal segmental glomerulosclerosis (FSGS). FSGS can occur as a primary or secondary disease. Primary FSGS can be further subdivided into idiopathic disease or familial (monogenetic) forms. FSGS may present with a spectrum of disease ranging from asymptomatic moderate-level (500 to 2,000 mg/day) proteinuria to symptomatic disease with frank nephrosis. It carries a worrisome long-term prognosis with a significant proportion of patients progressing to end-stage renal disease (ESRD). Glomerulopathies that classically present with nephrosis such as minimal change disease (MCNS) or membranous nephropathy are rarely asymptomatic.

Systemic Disease: Proteinuria manifesting as albuminuria may be a renal manifestation of systemic disorders. Renal involvement occurs in about two-thirds of patients with systemic lupus erythematosus (SLE).⁵ In those with isolated low-level proteinuria, this may be a sign of mild glomerular disease. However, in patients with nephrotic range proteinuria, this likely represents membranous disease. There is growing evidence that onset of SLE during adolescence and young adulthood is associated with more severe renal disease at presentation.⁶

Monogenetic Disorders: There are a group of rare genetic disorders associated with the occurrence of moderate to nephrotic range proteinuria that present in adolescence. One example is Fabry disease, which is associated with low- to moderate-level proteinuria in addition to other diverse manifestations of the disease.

Tubulointerstitial renal disease: Freely filtered LMW proteins are exclusively reabsorbed in the proximal tubule. As such, diseases causing generalized proximal tubular dysfunction may lead to low- to moderate-grade proteinuria called renal Fanconi syndrome. Fanconi syndrome may manifest secondary to (1) monogenetic disorders such as cystinosis, tyrisonemia, Wilson disease, Dent disease, or other inborn errors of metabolism or (2) an acquired condition due to reaction to penicillamine, aminoglycosides, heavy metals (mercury or lead), tenofovir, or other medication.