PDW (Platelet Distribution Width) shows how much platelets differ in size. It is an index of platelet anisocytosis: the greater the spread, the higher the PDW.
PDW is part of the complete blood count together with the mean platelet volume (MPV) and the plateletcrit. On its own it is non-specific, but it helps clarify the cause of platelet changes.
Reference ranges
The reference range depends on the analyzer; PDW is expressed in femtoliters (fL) or percent:
| PDW | ≈ 9 – 17 fL |
| In percent | ≈ 10 – 18% |
| Assessed with | MPV and platelet count |
What an elevated level means
A raised PDW means the platelets are of varying size — usually because of active turnover. It occurs with platelet activation and destruction, thrombosis, myeloproliferative diseases, sepsis, inflammation and some tumours. A high PDW together with a high MPV points to young, large platelets.
What a low level means
A low or normal PDW points to platelets that are uniform in size. Reduced values occur with bone-marrow suppression, when platelets are few and identical. An isolated normal PDW usually has no independent significance.
How the test is done
PDW is calculated automatically during a regular complete blood count — nothing separate needs to be tested. The parameter is assessed as part of a set: platelet count, MPV, plateletcrit and inflammatory markers.
What affects the value
- The type of analyzer and the units of measurement (fL or %) change the absolute figures.
- Microclots and platelet activation in the tube distort the result.
- A delay between the draw and the analysis affects platelet size.
Frequently asked questions
What does a high PDW with normal platelets mean?
It points to an increased spread of platelet sizes with an adequate platelet count. Often this is a non-specific finding, but it sometimes accompanies inflammation, platelet activation or early bone-marrow changes. It should be assessed together with MPV and the clinical picture.
Does an altered PDW need treatment?
PDW by itself is not a diagnosis and is not treated. It is an auxiliary index that helps a doctor interpret platelet changes. What matters is the cause (inflammation, bone-marrow disease, thrombosis) and the platelet count, not the isolated PDW figure.