To Perform Qualitative Tests for Urinary
Proteins^8
8.1 Theory
Proteins are organic compounds containing nitrogen in addition to carbon, hydro-
gen, and oxygen. Sixteen percent of total protein weight is contributed by nitrogen.
Proteins are responsible for distribution of bodyfluids and ions on both sides of the
membrane. The plasma proteins act as buffers and regulate change in plasma
pH. The normal protein excretion in urine is about 30–150 mg/day. The proteins
in urine are normally derived from plasmafiltrate and the lower lining of urinary
tract due to tissue damage. The serum globulins, albumin, and proteins secreted by
the nephron forms normal urinary proteins. About one third of urinary protein is
albumin and remaining includes many small globulins. Plasma proteins of molecular
weight less than 50 kDa easily pass through glomerular membrane of nephron and
are usually reabsorbed from kidney tubular cells. Albumin with molecular weight of
66 kDa is filtered only in very small amounts. Retinol-binding proteins,
β 2 microglobulins, immunoglobulin light chain, and lysozyme are also excreted in
small amount. In summary, a healthy man excrete trace amount of proteins in urine.
The large excretion of proteins in urine is called proteinuria. Proteinuria is an
important indicator of kidney disease and the risk of disease progression. The
proteinuria may also be caused by overflow of abnormal proteins in disease
conditions such as multiple myeloma. The prevalence of proteinuria increases with
kidney disease progression. The analysis of protein-to-creatinine ratio on a spot urine
sample is used commonly to assess the 24 h urine protein excretion, and an albumin-
to-creatinine ratio is used to approximate 24 h urine albumin excretion. Albumin-to-
creatinine ratio is more sensitive than protein-to-creatinine ratio in detecting low
levels of proteinuria. The normal urine albumin-to-creatinine ratio is less than
3.0 mg/mmol (ratio greater than 30 indicate severe increase of albumin excretion
in urine), while normal protein-to-creatinine ratio is 15 mg/mmol.
#Springer Nature Singapore Pte Ltd. 2018
V. Kumar, K. D. Gill,Basic Concepts in Clinical Biochemistry: A Practical Guide,
https://doi.org/10.1007/978-981-10-8186-6_8
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