Radioimmunoassay for urinary amyloid P component.

Area of Science:

• Biochemistry
• Clinical Chemistry
• Nephrology

Background:

• Amyloid P component (AP) is a serum protein found in all amyloid deposits.
• Quantifying urinary AP is important for diagnosing and monitoring amyloidosis and related kidney diseases.

Purpose of the Study:

• To develop and validate a sensitive radioimmunoassay for measuring amyloid P component (AP) in urine.
• To establish reference ranges for urinary AP excretion in healthy individuals.
• To investigate urinary AP levels in patients with rheumatic diseases and reactive amyloidosis.

Main Methods:

• A double-antibody radioimmunoassay technique was employed for AP quantification.
• Assay linearity, sensitivity, and precision (intra- and inter-assay coefficients of variation) were determined.
• Urinary AP excretion was measured in 20 healthy subjects and compared with patients with rheumatic disease and reactive amyloidosis.

Main Results:

• The radioimmunoassay demonstrated linearity from 5 to 2500 µg/L with a lower limit of 5 µg/L.
• Intra-assay and inter-assay coefficients of variation were low, ranging from 2.2% to 10.3%.
• Mean urinary AP excretion in normal subjects was 37 µg/24 hr. Patients with rheumatic disease showed increased excretion (mean 98 µg/24 hr; P < 0.005).
• Patients with reactive amyloidosis exhibited markedly increased diurnal AP excretion (mean 615 µg/24 hr).

Conclusions:

• A sensitive and precise radioimmunoassay for urinary AP has been established.
• Elevated urinary AP levels are indicative of rheumatic diseases and significantly higher in reactive amyloidosis.
• Urinary AP measurement is a valuable tool for assessing amyloidosis, particularly in the context of kidney involvement.