Background: To ensure consistency in manual urine sediment analysis, which is widely used in clinical practice, the standardization of high-power field (HPF) area is needed. Many laboratories use eyepieces with field numbers (FN), like 18, 20, and 22. However, because the HPF area varies by FN, the cells per HPF can vary. This study aimed to validate a standardized method for converting the cells per HPF counted using an FN 18 or 22 eyepiece to FN 20 eyepiece.
Methods: From September to October 2021, 100 red blood cell (RBC)-positive and 100 white blood cell (WBC)-positive specimens observed on the UF-5000 analyzer were used in this study. Urine sediment analyses were performed based on the modified Japanese Committee for Clinical Laboratory Standards guidelines and using FN 18, 20, and 22 eyepieces. The number of RBCs and WBCs per HPF was counted and categorized as RBC18, RBC20, and RBC22, and WBC18, WBC20, and WBC22. The ratio of the area between the eyepieces based on FN 20 was used as a conversion factor to convert RBC18c20 and RBC22c20, and WBC18c20 and WBC22c20, and these results were compared with RBC20 and WBC20, respectively.
Results: High correlations were found between RBC20 and RBC18c20 (y=1.0007x−0.1633; R²=0.999, P<0.001), RBC20 and RBC22c20 (y=1.0041x−0.3663; R²=9.998, P<0.001), WBC20 and WBC18c20 (y=1. 0004x+0.1421; R²=9.998, P<0.001), and WBC20 and WBC22c20 (y=0.9933x+0.3619; R²=9.998, P<0.001).
Conclusions: The conversion method based on the HPF area ratios could be used to standardize the cells per HPF although the eyepieces had different FNs.

Keywords: Urine, Sediment, Urinalysis, Standardization, Microscopy, High-power field, Field number