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Determination of white blood cell (WBC) estimation by blood film review is an important method for verifying manual and automated WBC counts, and for blood samples for which performing WBC counts is not feasible. While standardized methods are established for automated and manual WBC counts, the methods for performing WBC estimates from blood films are more anecdotal than analytical. Often, blood films are reviewed by various methods to arrive at the average number of WBCs per field of view (FOV), and this number is multiplied by a factor to estimate the WBC concentration. Error is introduced when protocols vary in the number of fields used to determine the average WBCs/FOV, differing diameters of FOV, and the choice of multiplication factor. Published methods have recommended calculating the average number of WBCs/FOV based on 10 fields (“field” sometimes qualified as hpf) and multiplying by a factor, for example, 2000 when using ×400 magnification [1]. The common feature among methods is the determination of the number of WBCs/FOV, yet microscope eyepiece diameters are not standardized and often differ between microscopes. Therefore, since FOV sizes are also inconsistent across objectives, the number of WBCs/FOV will be affected. The concept of FOV has largely been forgotten yet is critical to the accuracy and precision of WBC estimation from blood films. The microscope FOV diameter is calculated by the formula: DFV = FN/MO × MT, where FN is the field number stamped on the eyepiece, MO is the magnification of the objective, and MT is the magnification (if any) of the tube lens [2]. Assuming good quality blood film preparation, the WBC estimate quality is based on three concepts that affect accuracy compared to the true WBC count: the average number of WBCs/FOV, the diameter of the FOV, and the choice of the multiplication factor. The aim of this study was to compare WBC estimates performed at two FOV, using three methods for the multiplication factor, to the manual or automated WBC counts, respectively. The limits of agreement were based on the allowable total error for WBC of 20% and compared to the observed total error for method comparison studies [3, 4]. Blood samples from Kemp's ridley sea turtles (Lepidochelys kempii n = 39), green turtles (Chelonia mydas n = 3), and Atlantic bottlenose dolphins (Tursiops truncatus n = 30) were collected during wellness examinations at the National Aquarium. All sea turtle rehabilitation activities are conducted under USFWS Permit #ES70312D. Blood samples were analyzed within 1–4 h by experienced technologists for WBC counts by hemacytometer with the Natt & Herrick method (turtles) and by automated cell counter (dolphins), respectively. The three methods for multiplier factors included (a) the WBCs/FOV × objective power squared [5], (b) WBCs/FOV × factor calculated from 30 samples [6], and (c) WBCs/FOV × 2000 to represent a commonly used fixed factor regardless of FOV [1]. In the second method, the multiplication factor was determined from at least 30 blood samples where the WBC count for each sample was divided by the average WBCs/FOV by blood film, then averaging the 30 multiplication factors, and rounding for the final multiplication factor for that FOV6. For all three methods, the WBC estimate was determined based on 5 sets of 10 fields using 100× oil immersion for the sea turtles to better distinguish between lymphocytes and thrombocytes, and 40× objectives for the dolphin blood films. The FOV for the sea turtle samples at 0.18 showed observed total error (TEo) within or near the allowable total error (TEa) when the multiplication factor was the objective power squared or the calculated factor, but poor agreement using the fixed factor of 2000. Using the larger FOV at 0.265, only the calculated factor (3500) showed acceptable observed error. The results for dolphin samples showed the same pattern, suggesting that the microscope used for reporting the method of WBCs × objective squared in other studies likely had an eyepiece FN of 18 (Table 1). This study confirms the concept that a single factor, regardless of FOV, has poor correlation to the true WBC concentration and highlights the importance of determining the WBC estimate factor for each objective and FOV. The one-time effort required for 30 estimates with corresponding WBC counts and the simple mathematical calculation of averaging the 30 individual multiplication factors, rounded for the factor for that FOV, is considered worth the reward of having a reliable method for the WBC estimate for each available microscope in various diagnostic settings. The authors have nothing to report. The authors declare no conflicts of interest.