A patient presents with microcytic anemia, an elevated RBC count, and a Mentzer index of less than 13. What is the next appropriate investigation?

Appeared in: AIIMS Raipur NO - 2019 (Shift-2)

Explanation

The patient's constellation of findings—microcytic anemia, elevated RBC count, and a Mentzer index below 13—is classic for Thalassemia Trait.
The Mentzer index (MCV/RBC count) is a screening tool; a value less than 13 suggests that despite the red cells being small, their number is disproportionately high, a hallmark of thalassemia.
Hb (Hemoglobin) electrophoresis is the definitive diagnostic test used to identify and quantify different types of hemoglobin.
In β-thalassemia trait, Hb electrophoresis will typically show an elevated level of HbA2, confirming the diagnosis.

Option A: Serum ferritin is a marker of iron stores. It would be low in iron deficiency anemia (IDA), but the Mentzer index of less than 13 makes IDA less likely. In thalassemia trait, ferritin levels are usually normal or elevated.
Option B: A bone marrow biopsy is an invasive procedure that is not warranted for the initial investigation of a common microcytic anemia. It is reserved for complex cases, suspected malignancies, or aplastic anemia.
Option C: Serum transferrin, like ferritin, is part of the iron studies panel. It measures the main protein that binds and transports iron. Its level is typically increased in IDA, which is less likely here.

Clinical Relevance

Nursing practice connection: Use the key finding related to Differential diagnosis of microcytic anemia using laboratory indices to guide bedside assessment, documentation, and the next nursing action.
Correctly diagnosing thalassemia trait is crucial to avoid inappropriate and potentially harmful iron supplementation, which can lead to iron overload.
Patients diagnosed with thalassemia trait should be offered genetic counseling to understand the risk of having a child with a more severe form of thalassemia (e.g., thalassemia major) if their partner is also a carrier.
What if? If the patient's Mentzer index was 16, the suspicion would shift strongly towards Iron Deficiency Anemia. In that case, the most appropriate next investigation would be Serum Ferritin to assess iron stores.

How to Approach the Question

First, identify the core clinical problem from the stem: the patient has microcytic anemia.
Next, look for specific differentiating clues. The stem provides two critical ones: an 'elevated RBC count' and a 'Mentzer index of less than 13'.
Recall the main differential diagnoses for microcytic anemia: Iron Deficiency Anemia (IDA) and Thalassemia.
Apply the rule for the Mentzer index: a value less than 13 strongly points towards Thalassemia Trait, while a value greater than 13 suggests IDA.
Based on the high suspicion for thalassemia, determine the confirmatory test.
Select the option that represents the gold standard for diagnosing hemoglobinopathies, which is Hb electrophoresis.

Concept Tested & Keywords

Concept Tested: Differential diagnosis of microcytic anemia using laboratory indices.
Stem keywords: microcytic anemia, elevated RBC count, Mentzer index, less than 13
Lead-in keywords: next appropriate investigation
Clinical cues: Mentzer index < 13 is a strong indicator for Thalassemia Trait over Iron Deficiency Anemia.

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