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Mentzer Index as an Indicator of Hematological Health in Pregnant Populations: A Review

Emmanuel Ifeanyi Obeagu *

Department of Biomedical and Laboratory Science, Africa University, Zimbabwe

 

 

Introduction

Anemia is a global public health concern, affecting approximately 40% of pregnant women, with higher prevalence in low- and middle-income countries. The physiological changes in pregnancy often exacerbate pre-existing anemia or reveal underlying hematological disorders. Iron-deficiency anemia (IDA) and hemoglobinopathies such as thalassemia traits are the two leading causes, presenting a diagnostic challenge due to overlapping clinical and hematological features.^1-2 Maternal anemia, if left untreated, can lead to complications including preterm labor, low birth weight, and impaired cognitive and physical development in infants. Severe anemia is also associated with maternal mortality, especially in resource-constrained settings. Differentiating the type of anemia is critical, as the management of IDA (requiring iron supplementation) differs markedly from that of thalassemia traits, where iron overload could exacerbate complications.^3-4 The conventional diagnostic approaches for anemia include serum ferritin, hemoglobin electrophoresis, and genetic testing. While effective, these methods may not always be accessible or affordable, particularly in under-resourced healthcare systems. A simpler, cost-effective diagnostic tool is needed for initial screening to guide further investigations and management.^5-6 First introduced in the 1970s, the Mentzer Index has gained recognition as a practical indicator in differentiating IDA from thalassemia traits. It is calculated as the ratio of Mean Corpuscular Volume (MCV) to Red Blood Cell (RBC) count, parameters readily available in routine CBC tests. The simplicity and affordability of this calculation make it particularly appealing for use in pregnancy, where timely diagnosis is crucial.^7-8 In pregnant populations, the Mentzer Index serves as an effective screening tool for anemia, enabling healthcare providers to identify the probable cause early in prenatal care. This facilitates prompt and appropriate management, which is essential for minimizing risks to both mother and baby. Despite its utility, the Mentzer Index has limitations, such as reduced accuracy in mixed anemia cases and the need for population-specific threshold adjustments. ⁸ This review aims to evaluate the role of the Mentzer Index as an indicator of hematological health in pregnant populations. 

Hematological Health in Pregnancy

Pregnancy induces significant alterations in the hematological system to support the increased demands of the developing fetus and maternal tissues. Plasma volume expands by approximately 50%, leading to a relative hemodilution known as physiological anemia of pregnancy. Concurrently, red blood cell (RBC) mass increases, albeit at a slower rate than plasma expansion, contributing to decreased hemoglobin concentration. These changes are considered normal adaptations but can complicate the diagnosis of pathological conditions such as anemia.^9-10 Anemia is the most common hematological disorder in pregnancy, affecting about 40% of pregnant women globally, with a higher burden in low- and middle-income countries. Iron-deficiency anemia (IDA) accounts for the majority of cases due to increased iron requirements during pregnancy, inadequate dietary intake, and frequent gastrointestinal losses. Other causes include vitamin B12 and folate deficiencies, chronic diseases, and genetic hemoglobinopathies such as thalassemia.^11-12 Anemia during pregnancy poses significant risks to both the mother and the fetus. For the mother, severe anemia increases the likelihood of preterm labor, postpartum hemorrhage, and infection. For the fetus, it can lead to intrauterine growth restriction (IUGR), low birth weight, and developmental delays. These adverse outcomes underscore the importance of timely identification and management of anemia in pregnant populations.^13-14

Routine screening for anemia in pregnancy involves hemoglobin estimation, red blood cell indices, and peripheral blood smear analysis. Advanced diagnostics like serum ferritin, transferrin saturation, and hemoglobin electrophoresis are used to pinpoint the underlying cause. However, these methods may be inaccessible or unaffordable in resource-limited settings, necessitating simpler tools such as the Mentzer Index for preliminary assessment.¹⁵ The management of anemia depends on its etiology. While IDA requires iron supplementation, thalassemia traits necessitate avoiding iron overload, which can exacerbate oxidative stress and organ damage. Misclassification can result in suboptimal or harmful interventions, highlighting the critical need for accurate differentiation of anemia types in clinical practice.¹⁶ As a readily available and cost-effective parameter, the Mentzer Index has gained traction as a first-line screening tool for anemia in pregnancy. It is particularly useful in areas with limited access to advanced diagnostics, helping to distinguish between IDA and thalassemia traits. By enabling targeted management, the Mentzer Index contributes to better maternal and fetal health outcomes and reduces the burden of anemia in vulnerable populations.¹⁷

Application of Mentzer Index in Pregnancy

The Mentzer Index (MI) is calculated by dividing the Mean Corpuscular Volume (MCV) by the Red Blood Cell (RBC) count, both of which are components of routine complete blood count (CBC) tests. A Mentzer Index value greater than 13 suggests iron-deficiency anemia (IDA), while a value less than 13 indicates a likelihood of thalassemia trait. This simple formula enables healthcare providers to differentiate between these common causes of anemia without the need for specialized testing, making it particularly useful in prenatal care settings.^18-19 In early pregnancy, anemia is often asymptomatic and may go undiagnosed until complications arise. Utilizing the Mentzer Index during routine antenatal visits allows for early detection of anemia etiology. Early identification of IDA enables timely iron supplementation, while recognition of thalassemia traits can prompt genetic counseling and further evaluation of fetal health. This proactive approach minimizes risks of adverse outcomes for both the mother and the fetus.^20-21 The Mentzer Index is especially beneficial in low- and middle-income countries where advanced diagnostic tools, such as hemoglobin electrophoresis or genetic testing, may not be readily available. Its reliance on parameters from basic CBC tests makes it a cost-effective alternative for initial anemia screening. By streamlining the diagnostic process, MI ensures that limited healthcare resources are allocated efficiently, enabling better care for pregnant women.^22-23

Although the Mentzer Index is a valuable tool, its accuracy improves when used alongside other indices, such as red cell distribution width (RDW) and mean corpuscular hemoglobin (MCH). These parameters can help confirm the diagnosis and provide a more comprehensive understanding of the hematological profile. For instance, elevated RDW values are more indicative of IDA, while normal RDW with low MI strongly suggests a thalassemia trait.²⁴ Despite its utility, the Mentzer Index is not infallible. Mixed anemia cases, such as IDA coexisting with thalassemia, can yield ambiguous MI results, complicating diagnosis. Moreover, the index relies on population-specific reference values for MCV and RBC, which may vary with ethnicity, altitude, and dietary habits. These factors necessitate caution in interpreting MI results, particularly in diverse populations.²⁵ The application of the Mentzer Index in pregnancy significantly enhances maternal and fetal health outcomes by guiding appropriate interventions. Early and accurate diagnosis reduces risks associated with untreated anemia, such as preterm delivery, low birth weight, and postpartum complications. Additionally, the index supports better resource utilization by directing advanced testing and interventions to high-risk cases.²⁶

Limitations and Challenges

One of the significant limitations of the Mentzer Index (MI) is its reduced reliability in cases of mixed anemia, such as the coexistence of iron-deficiency anemia (IDA) and thalassemia traits. Mixed anemia can lead to intermediate Mentzer Index values, which do not clearly indicate either condition, complicating the diagnostic process. This challenge is particularly pronounced in populations with high prevalence rates of both IDA and hemoglobinopathies.

The effectiveness of the Mentzer Index depends on the accuracy of MCV and RBC reference ranges, which can vary significantly across different populations. Factors such as ethnicity, dietary habits, altitude, and genetic predisposition influence these parameters, making it difficult to establish universal cutoff values. Without population-specific adjustments, there is a risk of false positives or negatives, potentially leading to misdiagnosis.²⁴

The Mentzer Index is a screening tool rather than a definitive diagnostic method. While it is useful for differentiating between IDA and thalassemia traits, it does not account for other anemia causes, such as vitamin B12 deficiency, folate deficiency, or anemia of chronic disease. Clinicians must rely on additional diagnostic tools to identify these conditions, reducing the standalone utility of MI.²⁵

Physiological changes in pregnancy, such as hemodilution and increased plasma volume, can alter hematological parameters, particularly in the second and third trimesters. These changes may skew the Mentzer Index values, making it less reliable in diagnosing anemia during late pregnancy. This limitation underscores the importance of interpreting MI results in the context of gestational stage and other clinical findings.²⁶

The reliability of the Mentzer Index hinges on the accuracy of CBC measurements, particularly MCV and RBC count. In settings with suboptimal laboratory practices or outdated equipment, errors in these parameters can lead to incorrect MI calculations and potentially flawed diagnoses. This is a significant concern in under-resourced healthcare facilities.²⁷

There is no universal agreement on the cutoff values for the Mentzer Index, with thresholds varying slightly across studies and guidelines. Some researchers advocate for a stricter cutoff (e.g., MI < 12 for thalassemia), while others suggest broader ranges. This lack of consensus adds to the complexity of its clinical application, requiring clinicians to consider local population data and their experience.²⁷

Conclusion

The Mentzer Index stands as a simple, cost-effective, and accessible hematological tool for differentiating iron-deficiency anemia (IDA) from thalassemia traits, particularly in pregnant populations. Its reliance on basic complete blood count (CBC) parameters makes it an invaluable first-line screening method, especially in resource-limited settings. By facilitating early and accurate identification of anemia etiology, the Mentzer Index contributes to improved maternal and fetal outcomes, guiding appropriate interventions and preventing complications.

Conflict of Interest: Author declares no potential conflict of interest with respect to the contents, authorship, and/or publication of this article.

Source of Support: Nil

Funding: The authors declared that this study has received no financial support.

Informed Consent Statement: Not applicable. 

Data Availability Statement: The data supporting in this paper are available in the cited references. 

Ethics approval: Not applicable.

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