Red cell distribution width (RDW)：
- 代表紅血球大小的變異 -> 周邊血液抹片anisocytosis
- Width of the MCV histogram (bell-shaped curve around the mean)
高RDW = RBC 的size變化大，原因有：
- iron deficiency
- vitamin B12 or folate deficiency
- myelodysplastic syndrome (MDS)
- hemoglobinopathies(thalassemia, sickle cell…)
- anemia who have received transfusions
- microcytic cells from hemolysis ＋ a population of reticulocytes
註：就像麵包店一樣，好的麵包店做出來的麵包都是一樣的大小，RDW自然不高；偷工減料的麵包店做出來的麵包因為原料不足，自然大小不一，此時RDW就高起來了。因此可想而知，上述情形都會導致RDW升高：
- 原料不足：缺鐵性貧血、維他命B12或葉酸缺乏
- 麵包工人不良：骨髓造血疾病
- 麵包模具不良：hemoglobinopathies
- 混入其他店的麵包：輸血的病人
- 一些麵包被撕碎：溶血性貧血+網狀紅血球增生
正常範圍：11.5 to 14.5% -> 不是normal 就是 elevated
- RDW在正常範圍不代表正常
Ex: 正常RDW但都很小或都很大
- RDW高並不非常specific 或 sensitive：
Ex: 研究：26位沒治療的vitamin B12 deficiency有1/3是正常RDW
觀察性研究甚至發現高RDW 與不良預後有關：
- Overall mortality
- Cardiovascular/cerebrovascular events
- Venous thromboembolism
- Chronic kidney disease
- Liver disease
Reference:
UpToDate: Microcytosis/Microcytic anemia — RDW (size variability)
https://www.uptodate.com/contents/microcytosis-microcytic-anemia?search=red%20cell%20distribution%20width&source=search_result&selectedTitle=1~24&usage_type=default&display_rank=1#H5830437
