Simple method for correcting total protein in plasma for actual fibrinogen content

Using plasma instead of serum for routine chemistry analyses has many advantages. To overcome the disadvantage of inclusively measuring fibrinogen in the plasma total protein assay without changing the clinical significance of the total protein assay, we investigated the possibility of subtracting the actual amount of fibrinogen from the plasma total protein. The correlation between serum and plasma total protein was excellent (plasma total protein = 0.989 x serum total protein + 6.7 g/L; r = 0.969; n = 131; mean difference = 5.55 g/L; P < 0.001). When the plasma total protein was corrected for the actual amount of fibrinogen, the correlation with serum total protein was equally good but the intercept was practically eliminated (corrected plasma total protein = 1.009 x serum total protein + 0.25 g/L; r = 0.985; n = 131; mean difference = 0.78 g/L; P = 0.47). The mean concentration of fibrinogen was 2.5 g/L (range: 1.38-3.62 g/L; n = 404) for blood donors, 3.6 g/L (n = 2707) for patients from the outpatient department, 4.6 g/L (n = 2023) for patients admitted to the hospital, and 6.6 g/L (n = 219) for patients whose concentration of C-reactive protein was > 50 mg/L. We conclude that the plasma total protein result should be corrected for the actual amount of fibrinogen.