The “rule of three” in hematology offers a quick method to assess the accuracy of complete blood count (CBC) results, specifically red blood cell (RBC) count, hemoglobin (HGB), and hematocrit (HCT). It postulates that, in normocytic and normochromic red blood cells, HGB (g/dL) multiplied by 3 should approximately equal HCT (%), and RBC (millions/L) multiplied by 3 should approximately equal HGB. For instance, if HGB is 10 g/dL, then HCT should be around 30%. Significant deviations from this correlation may indicate instrument error or an underlying hematological abnormality.
The importance of this lies in its ability to serve as a quality control check, particularly in resource-limited settings or when immediate access to advanced diagnostic tools is unavailable. It provides a rapid means to identify potential inaccuracies, prompting further investigation and potentially preventing misdiagnosis. Historically, it has been a cornerstone of laboratory hematology, offering a basic yet crucial check on automated analyzer performance and the reliability of results before more sophisticated validation methods became widespread.
