Investigating the Reliability of HbA1c Monitoring for Blood Glucose Control During Late Pregnancy in Patients with Gestational Diabetes Mellitus (GDM) with and without β-Thalassemia Minor

Gestational diabetes mellitus (GDM), defined as hyperglycemia that is first discovered during pregnancy, is a high-risk condition for both mother and fetus. It is associated with adverse pregnancy outcomes, such as macrosomia (birth weight > 4000 g), shoulder dystocia, stillbirth, and neonatal complications, including hypoglycemia and respiratory distress syndrome [1]. The long-term adverse effects of GDM on the metabolism and cardiovascular systems of mothers and their children pose a significant threat to public health [2].

Patients with GDM require strict blood glucose control to reduce the incidence of perinatal complications in the mother or the infant. Glycated hemoglobin (HbA1c) is widely accepted to be the most reliable indicator of long-term glycemic control [3]. HbA1c is generated by the nonenzymatic binding of glucose to the N-terminal valine residue in a β chain of hemoglobin. The HbA1c level in an individual’s blood is proportional to the mean plasma glucose level over the lifespan of red blood cells (120 days) [4]. The Diabetes Control and Complications Trial provided extensive data on how the development and progression of complications depends on the HbA1c level in patients with diabetes mellitus [5].

HbA1c has also been used as an indicator of the blood glucose level in patients with GDM [6], but there is not enough clinical evidence to support its use in this context. Some studies have shown that HbA1c is affected by a variety of factors and thus cannot be used to accurately monitor blood glucose control during pregnancy [7]. HbA1c is a stable micro hemoglobin component that forms by the irreversible nonenzymatic reaction of glucose and hemoglobin in red blood cells [8]. The HbA1c concentration depends on plasma glucose levels and the RBC lifespan [9]. For example, it is known that the presence of hemoglobinopathy affects the results of almost any method of determining glycosylated hemoglobin [10].

The β-thalassemias are a group of hereditary hemoglobinopathies. They are caused by decreased or deficient synthesis of the β chains of hemoglobin, and result in different phenotypes ranging from severely anemic to asymptomatic individuals. The incidence of this disease is very high in southern China [11]. In southern Guangdong and Guangxi, 2.54% and 6.78% of the population are β-thalassemia carriers [12]. The severity of the thalassemia depends on the nature of the mutation. β-Thalassemia minor is caused by a mutation in one of two β-globin genes; this mutation does not severely damage the production of β chains of hemoglobin, so the patient may be relatively asymptomatic. A review of the literature indicates that it is currently unclear how to evaluate HbA1c in GDM patients with β-thalassemia minor. The purpose of the work performed in the present study was to investigate whether the HbA1c levels of GDM patients are affected by β-thalassemia minor and to subsequently discuss the limitations of HbA1c monitoring for blood glucose control.

The 134 participants in the study included 41 GDM patients with β-thalassemia minor (the study group) and 93 GDM patients without thalassemia (the control group). The clinical characteristics of the mothers and their newborns are summarized in Table 1. There was no statistically significant difference between the groups in newborn birth weight, maternal age, maternal prepregnancy BMI, gestational age, newborn sex, gravidity, and parity (P > 0.05 for each comparison; Table 1).

The blood glucose values of the participants in the study and control groups were measured at particular times during the gestation period. When the blood glucose values of the participants in the two groups were compared at each time point during the gestation period, there was no significant difference in the blood glucose values of the two groups (p > 0.05 for each comparison, i.e., at each time point; Table 2).

The rate of iron deficiency in the study group (12.19%) was significantly lower than that in the control group (58.06%); P < 0.05. The mean ferritin level in the study group (46.59 ± 18.03 ng/mL) was significantly higher than that in the control group (25.58 ± 11.42 ng/mL); P < 0.05 (Table 3). Also, a significant negative correlation was observed between HbA1c and ferritin in both the study group (R = − 0.459, P = 0.003) and the control group (R = − 0.358, P = 0.010); see Table 4.

Gestational diabetes mellitus (GDM), which has potentially adverse consequences, is a growing public health problem [16]. The prevalence of GDM in China has increased significantly in recent years [17]. Glucose control is the key medical intervention in cases of GDM.

The goal of glycemic control during pregnancy should be to keep blood sugar levels as close to normal as possible without inducing hypoglycemia. It is essential to find an accurate indicator of blood sugar, as this can provide valuable information for achieving glycemic control. Strict glycemic control should be implemented through self-monitoring of blood glucose (SMBG) or continuous glucose monitoring (CGM). It is difficult to perform blood glucose measurements in every patient, but blood glucose control can be monitored by measuring HbA1c. This approach is more specific and has lower biological and analytical variability than the fasting glucose test. There is a continuously increasing demand for the routine testing of HbA1c to detect diabetes mellitus and (especially) for long-term glycemic control [18].

Although the HbA1c reference interval for the general population is well established, the reference interval for healthy pregnant women is not clearly defined [19]. Different studies have reported different trends in HbA1c during pregnancy [20‐22]: HbA1c levels during pregnancy can gradually or very frequently increase or decrease. It is difficult to use HbA1c to monitor glycemic control in GDM patients due to the dramatic changes in physical condition that occur during pregnancy [23] and the different genetic backgrounds of pregnant women. Thalassemia is an inherited hemoglobin disease. Hemoglobinopathy can affect the HbA1c value, potentially leading to an inaccurate judgment of the patient’s blood glucose control [10].

The clinical manifestations of thalassemia are very diverse. The clinical classification scheme for β-thalassemia includes β-thalassemia major (TM; severe), β-thalassemia intermedia (TI; intermediate), and β-thalassemia minor (mild). TM refers to patients with severe anemia from the early stages of life on who require lifelong blood transfusion and iron chelation. TI patients are a highly diverse group who present various clinical severities: mild, moderate, and moderate-severe anemia. However, β-thalassemia minor is often asymptomatic or presents as mild anemia, and such patients do not require blood transfusion. Genetic counseling is needed. β-Thalassemia minor is caused by the presence of a single β-thalassemia mutation; the β-globin gene on the other chromosome is normal.

HbA1c is an indicator of hemoglobin glycosylation. The HbA1c concentration depends on the plasma glucose level and the lifespan of erythrocytes. Any condition that alters the lifespan of red blood cells may lead to an erroneous judgment of the HbA1c results [24]. It has been reported that the RBC lifespan is reduced in the presence of some variants of hemoglobin and in patients with thalassemia [25]. Therefore, the clinical significance of HbA1c in thalassemia patients may be misinterpreted.

This study found that GDM patients with β-thalassemia minor (the study group) showed no difference in blood glucose values from GDM patients without thalassemia (the control group) at various times during the gestation period, but that the HbA1c values of the study group were lower than those of the control group in late pregnancy. β-Thalassemia is an interfering factor when attempting to evaluate the level of HbA1c in GDM patients. This may be because β-thalassemia minor is potentially a hemolytic disease. We speculate that immature red blood cells are compensatory in β-thalassemia. The glycosylated hemoglobin of immature red blood cells is lower than that of mature red blood cells, causing HbA1c to decrease.

The HbA1c value of GDM patients with β-thalassemia minor is lower than that of GDM patients without thalassemia. This may be due to the iron load. When there is iron deficiency without blood loss, the lifespan of red blood cells is prolonged, increasing the saccharification reaction time and increasing the level of glycosylated hemoglobin [26]. This study found that there was a negative correlation between serum ferritin and HbA1c in both the study group and the control group. Compared with the control group, the incidence of iron deficiency in the study group was significantly lower, while the serum ferritin content was significantly higher. However, iron overload was not observed in the study group.

Iron demand increases during pregnancy. Iron deficiency is a common phenomenon among pregnant women, especially those in late pregnancy [27]. The same is true of GDM patients. In our study, the incidence of iron deficiency in GDM patients without thalassemia was 58.06% in late pregnancy. Studies have shown that normal pregnant women and diabetic patients have higher HbA1c levels in the third trimester [21]. Most GDM patients benefit from blood glucose control. The level of fasting blood glucose (FBG) tends to decrease during pregnancy. Thus, if HbA1c was only a reflection of the average blood sugar level, it would decrease as pregnancy progresses. This suggests that other factors besides the blood glucose level can affect the HbA1c value. This interference may be due to the increased likelihood of iron deficiency late in pregnancy.

β-Thalassemia minor may inhibit the level of serum hepcidin [28]. Hepcidin is considered to be a key regulator of the iron balance [29]. It is a negative regulator of the absorption of iron from the intestine and the release of iron from macrophages. Suppressing hepcidin can enhance iron absorption. Patients with β-thalassemia minor characteristically present a slow-moving increase in iron loading [28]. There may be hitherto unrecognized mechanisms for more effectively adapting to chronic anemia. In this study, the incidence of iron deficiency in GDM patients with β-thalassemia minor was only 12.19% during late pregnancy.

In summary, thalassemia, a hemolytic disease, leads to a shortened hemoglobin lifespan. Any condition that alters the lifespan of red blood cells may influence the HbA1c value. Moreover, thalassemia promotes iron load levels in pregnant women, and iron load is negatively correlated with HbA1c. Both of these causes led to lower levels of HbA1c in GDM patients with β-thalassemia minor than in those without thalassemia.

Maybe we should consider modifying the reference interval for HbA1c according to factors such as gestational age, ferritin level, hemoglobin level, and whether or not thalassemia is present. However, this would be very difficult because there are so many factors that affect the HbA1c value during pregnancy. Since the reference interval would therefore be rather complicated to determine, it would be hard to apply it clinically. While we continue to search for a more appropriate indicator of blood glucose level than HbA1c, in order to avoid misjudgments, we must approach clinical test results and literature reports based on HbA1c data with caution when they relate to pregnant patients, and we must identify all of the factors that interfere with the determination of HbA1c.

To properly manage GDM, it is critical to find suitable indicators for monitoring blood glucose control. At present, HbA1c is widely used for monitoring blood glucose control and for diabetes diagnosis. However, the HbA1c level is affected by many factors. Serum HbA1c values in GDM patients with β-thalassemia minor may suggest misleadingly low blood glucose levels. Therefore, care should be taken when using HbA1c for glycemic control assessment. We need to find more reliable indicators.

There are several limitations of this study. First, the HbA1c values of the GDM patients with β-thalassemia minor may have been influenced by factors not considered in this study. More detailed investigations of this issue are needed, such as prospective clinical trials and studies using animal models. Second, more cases and more controls are needed to examine this topic in more depth; for example, to explore the degree to which HbA1c is underestimated in patients with β-thalassemia minor and poorly controlled GDM. In this way, the limitations of HbA1c monitoring for blood glucose control can be probed in detail.

The HbA1c level is affected by various factors during pregnancy, making it difficult to establish a reference interval for pregnant women. Thalassemia is an inherited hemoglobin disease, and hemoglobinopathy can result in misleadingly low HbA1c values. Therefore, HbA1c is not a suitable indicator for monitoring blood glucose control in pregnant women, especially GDM patients with β-thalassemia minor.