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Ratio of serum amyloid A to C‐reactive protein is constant in the same patients but differs greatly between patients with inflammatory diseases

C‐reactive protein (CRP) is commonly monitored to track the activity of inflammation and has become the gold standard in the management of all inflammatory diseases. Indeed, serum amyloid A (SAA) have seemed to correlate moderately with CRP, but the discrepancy of CRP and SAA levels has often been reported, especially in rheumatoid arthritis. Then, we examined CRP reflects a real magnitude of inflammation in patients with rheumatic and infectious inflammatory diseases. A total of 414 patients with infectious and non‐infectious inflammatory diseases were enrolled. At initial visit, each patient underwent a clinical assessment and had also laboratory tests such as SAA and CRP. In each patient, we carried out a longitudinal analysis of CRP and SAA levels. We determined the inter‐individual correlation between SAA and CRP and also clarified intra‐individual changes of SAA/CRP ratio. SAA and CRP levels changed approximately linearly over time within individuals irrespective of rheumatic and infectious inflammatory diseases. However, SAA/CRP ratios differed dramatically between patients (from 0.117 to 50.8, median 5.71). In patients with high SAA/CRP ratio (>8.44), SAA is a better predictor of inflammation than CRP. In contrast, CRP is a better predictor in patients with low ratio (<3.52). Our results suggest that the SAA/CRP ratio differed greatly between individuals but was constant in intra‐individuals. Low CRP levels could be accompanied by SAA levels predicting any degree of inflammation, implying that CRP is not reflecting a real magnitude of inflammation. To evaluate the real magnitude of inflammation, to access the SAA/CRP ratio in advance is essential.