Urine is formed by ultrafiltration of plasma across the glomerular capillary wall. Proteins with a relative molecular mass >40 000 are almost completely retained, while smaller substances easily enter the glomerular filtrate. Most CSF protein originates by diffusion from plasma across the blood-CSF barrier. Elevated levels occur as a result of increased permeability of the blood-CSF barrier or with increased local synthesis of immunoglobulins2. Measurement of urine protein is becoming increasingly important in the detection of renal pathology3.
Proteinuria can occur in increased glomerular permeability, defective tubular reabsorption and abnormal secretion of protein into the urinary tract4. Albuminuria has been recognized as an early indicator of renal damage in diabetes that can be reversed if detected and treated sufficiently early5. The measurement of CSF total protein and specific protein is used to detect increased permeability of blood/brain barrier (the capillary endothelium of vessels of the central nervous system) to plasma proteins or to detect increased intrathecal secretion of immunoglobins3.
Protein measurements in urine are used in the diagnosis and treatment of disease conditions such as renal or heart diseases, or thyroid disorders, which are characterized by proteinuria or albuminuria. CSF protein measurements are used in the diagnosis and treatment of conditions such as meningitis, brain tumors, and infections of the central nervous systems2.