Validating the measurement of red blood cell diameter in fresh capillary blood by darkfield microscopy: A pilot study

ObjectivesAnalysis of fresh capillary blood using darkfield microscopy involves measuring red blood cell (RBC) diameter to assess for microcytosis or macrocytosis. The reference range for normal RBC diameter currently used in fresh capillary blood analysis by darkfield microscopy (FCB-DM) is the same as that used in haematology, i.e. 6.2–8.2 μm. However, the haematology reference range for RBC diameter refers to stained and dried blood specimens. Evidence from early research suggests that fresh RBCs are larger than stained and dried RBCs. The following article presents methodology and findings for the objective measurement of RBC diameter using the FCB-DM technique.MethodsFCB-DM screenings of 14 consenting participants were photographed to permit a quantitative analysis of RBC size and morphology. Participants also provided a venous blood sample for pathology testing, which included markers for RBC size. The researcher was blinded to the pathology results until all FCB-DM data analysis was complete.ResultsAll participants had a normal reference range for all pathology markers tested. The mean RBC diameter, determined by FCB-DM, was 8.51 μm (s = 0.161, s2 = 0.026, range = ±0.656) and the mode was 8.55 μm.ConclusionThese findings support evidence from early studies of RBC diameter, that fresh blood cells observed with darkfield microscopy are approximately 1.3 μm larger than dried and stained RBCs. Further research using a larger sample size is recommended.