William Aird Profile picture
Feb 21, 2023 9 tweets 4 min read Read on X
1/9

@jlberrymd #b12deficiency

Here's a cool case with an important take-home message.

A 48 yo M is referred to you with macrocytic anemia. Before looking at the graphic you should have a rough differential diagnosis at your finger tips!
2/9

Peripheral smear shows large RBCs, anisocytosis but otherwise unremarkable.

You work up the patient for macrocytic anemia by checking B12/folate and copper levels, liver function, thyroid function, SPEP but you stop short of a bone marrow biopsy. See graphic for results.
3/9

All lab tests are negative with the exception of a (VERY) elevated LDH, an increased AST:ALT ratio and low haptoglobin. A CT abdomen was normal (as was the PT/INR and serum albumin).

What is this suggestive of?
4/9

The results are suggestive of hemolysis.

Yet, the retics are low.

... and the LDH is disproportionately high.

High suspicion for ineffective erythropoiesis secondary to B12 deficiency!
5/9

OK, now a little Hx and Px 😉:

On Hx, the patient complained of fatigue and shortness of breath on exertion. There was no past history of abdominal surgery. He was not a vegetarian. Px exam revealed glossitis. CNS status was normal.
6/9

Serum B12 levels are NOT 100% sensitive for diagnosing B12 deficiency. If clinical suspicion is high, the next step is to check for functional B12 deficiency by measuring serum MMA and HCY (per BSH guidelines [and common sense]).
7/9

The MMA (more specific than HCY) was astronomically high, clinching the diagnosis of functional B12 deficiency. Anti-intrinsic factor antibodies were positive, c/w a diagnosis of pernicious anemia. The patient received B12 therapy and his labs values normalized.
8/9

It is not clear why some patients with functional/cellular B12 deficiency have normal serum B12 levels.

In most cases, it is likely related to increased binding of B12 to haptocorrin at the expense of transcobalamin II (the functional B12 transport protein).
9/9

In any event, the take home message is:

DO NOT DISCOUNT B12 DEFICIENCY BECAUSE THE SERUM B12 LEVEL IS NORMAL!!!

For infographic on B12 deficiency, see:

thebloodproject.com/copy-pa/

For case study of B12 deficiency, see:

thebloodproject.com/cases-archive/…

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More from @WilliamAird4

Feb 13
1/9

I’ve long taught a simple framework for hypoproliferative normocytic anemia that I still love:

• nutritional deficiency
• inflammation
• organ dysfunction (kidney, liver, endocrine, marrow)

Clinically useful. Memorable. Teachable.

And it still holds up. Image
2/9

But conceptually, something interesting is happening.

These categories aren’t actually the same kind of explanation.

Nutritional deficiency and inflammation are mechanisms.
Organ dysfunction is different.

And even within organs, not all behave the same way.
3/9

Take kidney disease.

We often say “anemia from CKD” as if that explains it.

And in a way, it almost does.

Because kidney disease sits very close to mechanism:
↓ EPO → ↓ erythropoiesis

It’s only one step away.
Read 9 tweets
Feb 13
1/11

I posted a poll:

A woman with ferritin 10 and Hb 12.2 (baseline 14). How should this be described?

Here’s how you answered:

• non-anemic Fe deficiency: 35%
• Fe deficiency anemia: 32%
• Fe deficiency with relative anemia: 27%
• none: 6%

Really interesting spread! Image
2/11

This tells us something important: clinicians sense a mismatch between definition-based language and physiology-based thinking, even if we disagree on terminology.
3/11

By strict WHO criteria, she is not anemic.
Hb ≥12 in women = normal.

So formally the correct label is: iron deficiency without anemia.
Read 11 tweets
Feb 10
RETICS IN PV

I recently ran a poll asking whether polycythemia vera (PV) is associated with:

• ↑ absolute reticulocyte count
• ↑ % reticulocytes
• both
• neither

The most popular answer was also the best answer: ↑ absolute reticulocyte count. Image
2/10

At first glance, that can feel counterintuitive.

PV is a disease of excessive red cell production, so why wouldn’t the reticulocyte percentage also be increased?

The answer depends on what reticulocytes actually measure.
3/10

Two metrics that are often conflated:

• Absolute reticulocyte count = how many reticulocytes are circulating

• Reticulocyte percentage = reticulocytes ÷ total red cell mass

They answer different physiologic questions.
Read 11 tweets
Feb 2
1/13

I posted a poll asking:

In acute GI bleed anemia, would you give 1 g IV iron regardless of ferritin?

Results:

• 27% yes — anticipate iron debt
• 12% sometimes
• 21% only if ferritin is low
• 41% no
2/13

First, an important acknowledgment:
There is no right answer here.

There are no firm guidelines that tell us what to do in this situation. Reasonable clinicians land in different places.

This is a gray zone where physiology, timing, and judgment matter.
3/13

So rather than argue what we should do, I want to walk through the numbers and biology and explain why some clinicians anticipate iron debt even when ferritin is normal.

Let’s take a concrete example.
Read 13 tweets
Jan 23
1/9

Yesterday I posted a CBC + reticulocyte count and asked for your diagnostic thoughts. Many of you offered great reasoning. The correct diagnosis was hemoglobin C disease.

Let’s unpack why this case is such a good learning example. 👇 Image
2/9

Microcytosis often triggers a reflex binary:
iron deficiency vs thalassemia trait.

That’s a useful starting point. But it’s incomplete. Structural hemoglobin variants (like HbC and HbE) also belong on that list.
3/9

Several people calculated the Mentzer index (MCV/RBC):

75 / 4.0 ≈ 18 → “suggests iron deficiency (ID).”

Important teaching point:

The Mentzer index was designed to distinguish thal trait vs ID. It is not validated for structural hemoglobinopathies like HbC or HbE.
Read 9 tweets
Oct 21, 2025
1/7

A STRUCTURED APPROACH TO NORMOCYTIC ANEMIA

A practical, population, and molecular look at how we reason through normocytic hypoproliferative anemia at the bedside.

(Full TBP tutorial linked below ⬇️)
2/7

1️⃣ The ABC Spine – A Bedside Framework

A: substrate supply (nutritional)
B: regulatory signaling (inflammatory)
C: organ dysfunction (kidney, liver, endocrine, marrow)

We focus here on the hypoproliferative end of the spectrum, where the marrow is not keeping up. Image
3/7

2️⃣ Anemia of the Elderly – A Population Perspective

In older adults, causes divide roughly:

1/3 nutritional 
1/3 inflammatory
1/3 unexplained

Each slice maps back to A, B, or C of the spine. Image
Read 7 tweets

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