PDW blood test: what it means and what the result shows

PDW is one of the platelet indices that the analyser calculates automatically from the same sample used to measure the platelet count (PLT) and the mean platelet volume (MPV) in a full blood count. The analyser builds a curve of platelet size (volume) — a histogram: MPV shows the average of this curve, while PDW shows how wide the curve itself is, that is, how much platelet sizes differ from each other. When all the platelets are of a similar size, the curve is narrow and PDW is low.

When the blood contains platelets of the usual size along with unusually large or small ones — for example, when the bone marrow is hurrying to produce new, larger platelets because platelets are being destroyed faster in the body — the curve widens and PDW rises. This is why PDW is rarely assessed on its own: it is interpreted together with PLT and MPV, because this combination helps to tell whether a change in the platelet count is caused by impaired platelet production in the bone marrow, or by their increased destruction or consumption in the body.

• As part of a full blood count during a routine health check — PDW is included automatically along with PLT and MPV
• When a full blood count has found an abnormal platelet count (PLT) — PDW and MPV help to assess the possible cause
• When bleeding from wounds takes a long time to stop, or nosebleeds are frequent
• When bruises appear easily with no clear cause
• When monitoring a known blood disease or the course of its treatment over time

In the widely available sources (MedlinePlus, NHS) no specific numeric PDW reference interval with units could be found — the reference interval depends on the laboratory's analyser model. You will find the exact PDW interval on your own blood test report, next to the result.

A raised PDW

PDW and MPV are calculated from the same platelet size distribution curve, so a raised PDW is most often seen together with a raised MPV — platelets of differing sizes are circulating in the blood, among them larger, younger ones. According to MedlinePlus, a higher MPV (and with it a wider PDW) is caused by conditions in which the body destroys or uses up platelets faster than usual.

Possible causes of a raised PDW:

• Thrombocytopenia (a reduced platelet count) — because of their increased destruction in the body, when the bone marrow compensates by producing new platelets of differing sizes
• Myeloproliferative diseases (bone marrow disorders in which too many blood cells are produced)
• Pre-eclampsia during pregnancy
• Heart disease
• Diabetes mellitus
• Haemolytic anaemia (increased breakdown of red blood cells)

What to discuss with a doctor:

• The platelet count (PLT) and the mean platelet volume (MPV) — together with PDW they help to assess the cause more precisely
• A review of the overall blood picture — whether there are any other abnormalities
• Whether there are signs of bleeding or bruising
• When it is worth repeating the test, so that the change over time can be seen

A low PDW means that the platelets in the blood are more similar in size to each other. Since PDW and MPV are calculated from the same curve, a low PDW is most often seen together with a low MPV — this may indicate slower, more uniform platelet production in the bone marrow.

Possible causes of a low PDW:

• Some cancers that affect how the bone marrow works
• The side effect of certain medicines on the bone marrow
• Aplastic anaemia (impaired blood formation in the bone marrow)
• Autoimmune diseases
• Bacterial or viral infections
• Alcohol use disorder

What to discuss with a doctor:

• The platelet count (PLT) — whether there is thrombocytopenia
• A full blood count and an assessment of bone marrow function, if the abnormality persists
• The medicines being taken — whether any of them could affect platelet production (to discuss with your doctor)
• When it is worth repeating the test to monitor the change over time

How PDW differs from MPV

Both values are calculated from the same platelet size distribution curve: MPV shows the mean platelet size, while PDW shows how wide the distribution of sizes itself is, that is, how much the platelets differ from each other in size. They are most often assessed together. The cause of a raised or a low PDW is established by a doctor after assessing the whole blood picture.