The “Thomas Plot”: Its Usefulness in the Assessment of Iron Status in Anemia.

A diagnostic plot to assess iron status in anemia of chronic disorders (ACD) has recently been developed (Thomas C, Thomas L. Lab Hematol 2005). The plot is based on the relation between the ratio of the soluble transferrin receptor value to the logarithm of the ferritin value (sTfR/log sFt) and the RET-He parameter -reticulocyte hemoglobin equivalent-. A different cut-off point for sTfR/log sFt is proposed for patients with a CRP ≤5mg/L or with a CRP >5mg/L. The main aim of our study was to validate the usefulness of the “Thomas Plot” in assessing iron status. We also ascertained whether a modification on the sTfR/log sFt cut-off point improved the results. CRP was determined in 306 patients, 198 with iron deficiency (ID) and 108 with ACD. All of them were anemia cases studied in accordance with our standard protocol. RET-He, sTfR and sTfR/log sFt were calculated in all samples. The Thomas Plot was applied to all the cases and the results were compared. A discrepancy was considered relevant when it implied a change in patient treatment. Using logistic regression analysis, we investigated the optimum cut-off point of sTfR/log sFt in our series. Overall, the Thomas Plot correctly classified 223/306 patients (73%). There were clinically relevant discrepancies in 29 patients (9.5%), in 5 patients with CRP ≤5mg/L (5%) and in 24 patients with CRP >5mg/L (12%). It failed to identify ID in 12/198 patients (6%) but classified 17 cases with ACD (16%) as latent ID or IDA. The optimum sTfR/log sFt cut-off point in our series was 2.4 for patients with CRP ≤5mg/L and 2.39 for patients with CRP>5mg/L. As a consequence, we investigated a modified “Thomas Plot” using a sTfR/log sFt cut-off point of 2.4 for all the patients. The modified algorithm correctly classified 77% of the patients. There were clinical relevant discrepancies only in 17 patients (5%). It failed to identify ID in 12/198 patients (6%) and only classified 5 cases with ACD as latent ID or IDA (4.6%). The sensitivity of the Thomas Plot to identify ID was very high but the specificity was lower. A cut-off point of 2.4 for sTfR/log sFt is useful for patients with or without inflammatory traits and yields more accurate results.