A 32-year-old female presents with a microcytic anemia: what’s the diagnosis?

Here’s a practice question on anemia – see if you can get the right answer before scrolling down to the discussion.

A 32-year-old female presents for a routine physical. Her CBC shows the following:

Hgb 9 g/dL (12-16)

MCV 72 fl (80-100)

RBC 6.4 x 10¹²/L

RDW 11.8% (12 – 13.5)

What is the most likely diagnosis?

A. Iron-deficiency anemia

B. Thalassemia

C. Megaloblastic anemia

D. Autoimmune hemolytic anemia

E. Microangiopathic hemolytic anemia

(keep scrolling down to see the correct answer)

The correct answer is B, thalassemia. So how come it’s thalassemia, and not iron-deficiency anemia (IDA), considering that the patient is a female of child-bearing age (in whom IDA might be more likely than thalassemia)?

There are a couple things you can look at to tell apart a typical case of IDA from a typical (mild-moderate) case of thalassemia: the RBC and the RDW.

The RBC is decreased in IDA (which makes sense – there’s not enough iron, so the body doesn’t make as many red cells). But it’s actually increasedÂ in thalassemia, for reasons that aren’t entirely clear. It’s not a really logical thing – it just happens to be the case in thalassemia that the marrow makes an increased number of red cells, and they’re smaller than normal.

The RDW is increased in IDA, which makes sense. In IDA, as each new wave of red cells comes out, there’s less and less iron, so the cells get smaller and smaller (which means that there’s an increased range of sizes – the older red cells are closer to normal in size, but the newer ones are smaller) – so the RDW goes up.

The RDW tends to bedecreasedÂ in thalassemia – also for reasons that aren’t entirely clear! It’s easy to see why the RDW isn’t increasedÂ in thalassemia (you don’t have a changing situation like you do in IDA, with less and less iron over time – it’s just a constant decreased amount of alpha or beta chains). But why it would actually be decreasedÂ – why the body would make red cells that are all exactly the same size, even more so than in normal blood – doesn’t seem to make a lot of sense.

So in this question, you’re right to think that IDA is a more likely choice in a woman of reproductive age. However, the increased RBC and decreased RDW both point towards thalassemia.

In real life, of course, iron studies would be the definitive way to diagnose IDA, and Hgb electrophoresis would be necessary to diagnose thalassemia.

By the way: C (megaloblastic anemia) is incorrect because megaloblastic anemias are macrocytic. And D (autoimmune hemolytic anemia – which includes both warm and cold) and E (microangiopathic hemolytic anemia) are normocytic.