Background Currently, supplementary serological testing for β-D glucan (BDG) is often selected to diagnose deep mycosis in care covered by the health insurance in Japan. The Wako method used by our center has low sensitivity, and different studies have used different cut-off values due to factors that cause false positives and false negatives. One possible cause of false negatives is the use of platelet-rich plasma (PRP) as the sample material. Because phagocytic white blood cells (WBC) are precipitated by centrifugation and only plasma is measured, it seems unlikely that the actual amount of BDG is being measured when using PRP. Further, a frequent cause of false positives is contamination from blood products and gauze containing BDG. To resolve these issues, the blood cell separator, hydroxyethyl starch, is used to precipitate only the red blood cells to obtain leukocyte-rich plasma (LRP). We hypothesized that it might be possible to improve the diagnostic rate of deep mycosis by measuring the BDG content of plasma containing WBC and fungal components and by comparing the BDG content of PRP and LRP measured simultaneously. Materials and Methods Healthy human blood, albumin-added blood, wrung-out gauze fluid-added blood, and fungal solution-added blood were prepared, and PRP and LRP were prepared using hydroxyethyl starch. The BDG content of each sample was measured using the Wako method and compared. In addition, PRP and LRP of fungal-added blood were Gram-stained and examined under a microscope, and the number of WBCs and phagocytosed fungi was counted visually and compared. Results Measuring the BDG content of LRP confirmed that there were no false positives with LRP, and in vitro experiments comparing albumin-added false-positive blood to fungal-added blood showed significant differences between PRP and LRP only in the fungal-added blood. Conclusion Calculating the BDG-ratio (LRP/PRP) by measuring both LRP and PRP may eliminate false positives and false negatives of true deep mycosis and improve the diagnostic rate.