Proteinuria: A Guide to Diagnosis and Assessment

Sasha-Kay D’Aguilar and Amith Skandhan*

Proteinuria: A Guide to Diagnosis and Assessment

Normal total urine protein excretion is approximately <150
mg/day and any value above this is considered proteinuria.

In majority of cases, the proteinuria that is excreted inappropriately
is albumin. Normal daily albumin excretion is<30 mg/day
with the mean being 5-10 mg/day.

Standard urine dipstick test only detects albumin once level
is significantly elevated, usually at 300 mg/day.

Proteinuria has many clinical implications. It is used for the diagnosis,
staging, progression, and treatment response of chronic
kidney disease due to diabetic and non-diabetic etiologies.

Albuminuria is primarily used to evaluate diabetic kidney disease.
Proteinuria is also an independent marker of cardiovascular disease
and mortality.

There are four mechanisms by which proteinuria may occur: glomerular,
tubular, overflow and post-renal. Each mechanism tends
to result in a specific degree of proteinuria.

This is the most common cause of pathological proteinuria and
results from increase glomerular filtration of plasma proteins
due to altered glomerular capillary permeability.

Normally, the charge and molecular size selectivity of
the glomerular capillary wall prevents albumin, globulins and high
molecular weight proteins from crossing.

Capillary endothelial cells and glomerular basement membrane have a negative
charge owing to the presence of polyanions such as heparin sulfate

These polyanions repel other anions like albumin
The glomerular capillary walls also have functional pores
through the glomerular basement membrane that traps most large
proteins with a molecular size of >100 kDa.

Low molecular weight proteins <20 kDa are freely filtered.
Therefore, measuring urine albumin is more sensitive for detecting
changes in glomerular permeability.

These proteins include β-2 macroglobulin, immunoglobulin light chains,
apoproteins and polypeptides. Defects in the tubuloepithelial cells
result in increased tubular excretion of these proteins.

Intern Med Open J. 2020; 4(1): 3-9. doi: 10.17140/IMOJ-4-112