Quantitation of Serum C-reactive Protein and Its Change under Pathologic Conditions : (III)Extemely Low Levels in Various Disorders and Their Significance

Serum C-reactive protein (CRP) is usually determined as a parameter of inflammation. Highly-sensitive immunoassay for CRP was recently developed and found to be clinically useful in the early diagnosis of infection in neonates by monitoring low levels of CRP (<50μg/dl). We used the LPIA system (Latex Photometric Immunoassay) and the PAMIA system (Particle Counting Immunoassay) as highly-sensitive CRP assay methods. α_1-Antitrypsin (α_1AT), α_1-Acidglycoprotein (α_1AG), Haptoglobin (HP) and Complement C3 (C3) were determined by laser nephelometry. Interleukin 6 (IL-6) was determined by an enzyme immunoassay using a chemiluminescent method. In order to ascertain the usefulness of this sensitive assay for adults, we monitored CRP in 917 inpatients with various tumors, various cancers, collagen disease, renal disease, diabetes mellitus, etc. In some of these patients, the CRP level was too low considering their diseases. We found that CRP levels decreased after steroid therapy and that this decrease was dependent on the steroid dose: no significant decrease after 10mg/day dosage, a marked and rapid decrease to an extremely low level (<34μg/dl) after 40mg/day dosage. When patients under steroid therapy got infectious disease, CRP levels increased to, or above, the normal level. However, these increases were not so marked that serial CRP monitoring at intervals of 1 to 2 weeks was required for the diagnosis of infection in those patients. It was established that serial measurement by highly-sensitive assay was reliable for the early diagnosis and management of infection. In one patient with nephrotic syndrome and another patient with malignant lymphoma, CRP decreased, however, IL-6 and other acute phase reactants in serum, such as α_1AT, α_1AG, Hp and C3, IL-6 were unchanged after steroid therapy. The mechanism of this CRP decrease under steroid therapy is unclear. Although the antiinflammatory action of steroid may be responsible, from these results, it is considered that another mechanism may be involved.