To Perform Qualitative Tests for Urinary Proteins
Proteins are organic compounds containing nitrogen in addition to carbon, hydrogen, and oxygen. Sixteen percent of total protein weight is contributed by nitrogen. Proteins are responsible for distribution of body fluids 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. 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. 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.