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The Astrim is a noninvasive hemoglobin monitoring device that uses near-infrared radiation. We studied the usefulness of this device for patients with hematological disorders. We measured hemoglobin levels 309 times in 140 subjects, using an automated hematology analyzer (K-4500) and the Astrim. The coefficient of correlation between Hb levels with the K-4500 (K-Hb) and the Astrim (A-Hb) was r=0.591 (P<0.001). The coefficient of correlation between A-Hb and K-Hb from 174 specimens with anemia and 135 without anemia was r=0.531 (P<0.001) and r=0.345 (P<0.001), respectively. In the 309 specimens, the Astrim showed a diagnostic sensitivity and specificity for anemia of 78.3% and 69.0%, respectively. While the r-values, sensitivity, and specificity were passable, they appeared to be insufficient for an accurate evaluation. We believe this was caused by the measurement conditions (i.e., the finger selected for measurement, the finger position in relation to the CCD camera, and the finger temperature). On the other hand, the precision test results were good. Therefore, if careful attention is paid to measurement conditions, the noninvasive Astrim is clinically useful for continuous hemoglobin monitoring of hematological disorders and patients with hemorrhagic diseases, such as perioperative hemorrhage and gastrointestinal bleeding. However, it is difficult to determine Hb concentrations in one measurement using the Astrim because the rvalues, sensitivity, and specificity are insufficient for accurate evaluations of inpatients, particularly patients with advanced diseases. Therefore, we believe it is necessary to improve the system so that A-Hb can be accurately determined in one measurement.