# RDW blood test: what red cell distribution width means > Doctano turns the lab reports you already have into one private, source-linked health timeline, with an AI that answers from your own results. How uneven your red blood cells are in size, and what that unevenness can reflect. This is the Markdown version of https://doctano.com/tests/rdw. The HTML page is the canonical version. RDW, or red cell distribution width, describes how much the red blood cells in a sample vary in size. It is usually reported as a percentage and is read with MCV and the other red cell indices. The range printed on your report is the one that applies. Most blood count lines report an amount or an average. RDW reports a spread. That makes it useful in a different way, and it also makes it easy to read as a severity score when it is not one. ## What the test measures An analyser measures the volume of each red cell and builds a distribution curve from those measurements. RDW describes how wide that curve is. A narrow curve means the cells are similar in size; a wide curve means the sample contains a mixture of smaller and larger cells. Two versions appear on reports. RDW-CV expresses the width as a percentage relative to the average cell size, and it is the form most reports print. RDW-SD expresses the width in femtolitres as an absolute measurement of the curve. The two describe related but different statistics, and a value from one should not be compared with a range for the other. RDW is not a measure of cell size. MCV is the average size, and RDW is the variation around that average. A sample can have an average size inside the printed range and a wide spread, because small and large cells balance each other out in the mean. ## Why it may be ordered - As part of a complete blood count, where it is reported with MCV, MCH, and haemoglobin. - When anaemia is being assessed, because the spread helps a clinician narrow the possibilities. - When a low haemoglobin sits beside a normal MCV and the report needs another angle. - To follow a known blood condition or the response to a long-term medicine. - When reports from two laboratories are being compared and the indices do not match. It is rarely ordered on its own. RDW without MCV and haemoglobin would describe variation with nothing to anchor it. ## The unit on your report | What you may see | What it means | | --- | --- | | RDW, RDW-CV | The test name. RDW-CV is the percentage form that most reports print. | | % | The spread of red cell sizes relative to the average cell size. | | RDW-SD | A different statistic, reported in fL. It describes the width of the distribution itself, not a percentage. | | fL | Femtolitres, the unit used when RDW is printed as RDW-SD. | | MCV | The average cell size, printed nearby. RDW describes the spread around it, not the average. | ## Why this page does not print a range A reference range belongs to the laboratory, the method, and the group of people it was built from. Ranges can differ with age and sex, pregnancy often has its own, and the clinical context changes how a result is read. The analyser's algorithm for building the distribution differs between manufacturers, so two laboratories can report the same sample with different ranges and slightly different values. The range printed on the report that produced your value is the one that applies. Doctano keeps that printed range with the result and does not replace it with a general range. If a report prints RDW-SD rather than RDW-CV, read the range that matches that line. ## What a high value can mean A higher RDW means the cells vary more in size than the reference group used by the laboratory. Possible contributors include deficiencies that change how red cells develop, a mixture of two deficiencies, recovery after bleeding or treatment when young cells enter the circulation, recent transfusion, and some inherited haemoglobin conditions. Long-standing inflammation and some bone marrow conditions can also widen the spread. A wide spread does not identify which of these applies. It is read with MCV, haemoglobin, MCH, and the blood film, and a clinician decides what the pattern suggests. A high RDW does not confirm or rule out any condition. ## What a low value can mean A lower RDW means the red cells are more uniform in size than the reference group. It is not usually interpreted on its own, and it is not a sign of health or of anything else by itself. A very uniform population can occur in some conditions where red cell production is steady, and it can also appear when a sample is analysed in a particular way. Because a low RDW is not specific, the value is read with the rest of the count rather than treated as a finding in isolation. ## What can move the value - **Young red cells.** Reticulocytes are larger than mature cells, and a higher proportion of them widens the spread. - **Transfusion.** Donor cells mix with the person's own cells and can widen the distribution. - **Time before analysis.** Cells change in the tube, and a delayed sample can shift the curve. - **Analyser and algorithm.** Different instruments build and measure the distribution differently. - **Very high white cell or platelet counts.** Other cells can interfere with some counting methods. - **Medicines.** Some affect red cell development. A clinician can say whether one is relevant to a particular result. ## What to compare it with Read RDW with MCV, haemoglobin, MCH, and the red cell count on the same report. A high RDW with a low MCV points in a different direction from a high RDW with a high MCV, and a high RDW with an average MCV suggests a mixed population that the average hides. The blood film can show the difference between the cell populations directly. The most useful comparison is the previous RDW from the same laboratory on the same analyser. A change between two laboratories can reflect the method as much as the person. ## How to see the trend Doctano reads the RDW value, its unit, and the reference range printed on the report. It keeps that range with the result, normalizes the marker name across labs, and places the value on the profile's timeline. Each point still opens the report it came from, so the original page can be checked. RDW usually moves slowly, so a trend across reports is more informative than a single point. Verify the value against the original report before treating a change as real, especially when the two results came from different laboratories. ## What this page is not This page explains the test in general. It does not interpret your result, diagnose a condition, or recommend treatment. It is not medical advice. Verify the value and the unit against the original report, and discuss the result with a qualified professional. ## RDW questions **What does RDW measure?** It measures how much the red blood cells in a sample vary in size, not how large they are on average. A higher RDW means a wider spread of cell sizes. **What is the difference between RDW-CV and RDW-SD?** RDW-CV is reported as a percentage and describes the spread relative to the average cell size. RDW-SD is reported in fL and describes the width of the size distribution itself. The two are different statistics and should not be compared with each other. **Can RDW be high with a normal MCV?** Yes. MCV is an average, so a sample can contain small and large cells that balance out while the spread stays wide. RDW is the line that shows that variation. **Does a high RDW mean anaemia?** No. A wide spread of red cell sizes can be associated with several conditions, including some deficiencies and recent transfusion, and it does not confirm or rule out anaemia. It is read with haemoglobin, MCV, and the rest of the report. ## Limits **This is not a diagnosis.** RDW is read with your other results, your history, and the range printed on your report. 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