[Case of suspected pseudo-elevation of serum creatinine immediately after intra-articular injection into a wrist joint of a pharmaceutical preparation of dexamethasone containing creatinine as a buffer].

We report a case of suspected pseudo-elevation of serum creatinine in a patient with rheumatoid arthritis, whose blood sample was obtained from his right cubital vein immediately after the intra-articular injection of 0.67 mg of a pharmaceutical preparation of dexamethasone (Decadron) containing creatinine as a buffer into his right wrist joint to alleviate arthralgia. The 61-year-old male patient, who had been treated for rheumatoid arthritis, was referred to our outpatient clinic by his family doctor because of sudden elevation of his serum creatinine, which hitherto had been normal. Acute kidney injury (AKI) was suspected, but his medical history, physical examinations, and laboratory findings did not support the diagnosis of AKI. The preserved serum sample obtained when the elevation of his serum creatinine was reported was examined and revealed that the serum concentration of cystatin C was normal, but the serum concentration of creatinine measured with high-performance liquid chromatography method was elevated to a degree consistent with the serum creatinine concentration measured with enzymatic method. Accordingly, we predicted the possibility that creatinine contained in Decadron as a buffer was included in the measurement of his intrinsic serum creatinine. To test this hypothesis, the creatinine concentration of serum samples from another patient with known amounts of added Decadron were measured, and the additive effect of the extrinsic creatinine in the measurement of serum creatinine concentration was confirmed. These findings clearly suggest that we should consider the influence of extrinsic creatinine as an inactive ingredient in the measurement of serum creatinine.