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Impact of Altitude on the Accuracy of HbA1c in Reflecting Glycemic Status in Patients with Type 2 Diabetes: A Cross-Sectional Study in ChinaShipengGan1
PeiyingHu2
NaowuRenqing3
XueyanZhou1
ChunyanYan1
LiTian3
YananLi3
HongjunLv1
ShengyanMa1
YifeiYu4✉Emailyifeiyu09@fudan.edu.cn
1Department of Endocrinology and MetabolismThe Third People’s Hospital of XiningXiningChina
2Qinghai Vocational And Technical Institute of Animal Husbandry And VetQinghaiChina
3Department of Endocrinology and MetabolismHainan Tibetan Autonomous Prefecture Hospital in Qinghai ProvinceQinghaiChina
4Department of Endocrinology and Metabolism, Huashan HospitalFudan UniversityShanghaiChina
Shipeng Gan1*, Peiying Hu2*, Naowu Renqing3*, Xueyan Zhou1, Chunyan Yan1, Li Tian3, Yanan Li3, Hongjun Lv1, Shengyan Ma1, Yifei Yu4#
1 Department of Endocrinology and Metabolism, The Third People’s Hospital of Xining, Xining, China
2 Qinghai Vocational And Technical Institute of Animal Husbandry And Vet, Qinghai, China
3 Department of Endocrinology and Metabolism, Hainan Tibetan Autonomous Prefecture Hospital in Qinghai Province, Qinghai, China
4 Department of Endocrinology and Metabolism, Huashan Hospital, Fudan University, Shanghai, China
#Corresponding author
Yifei Yu, Email address: yifeiyu09@fudan.edu.cn
Shipeng Gan, Peiying Hu and Naowu Renqing contributed equally to this work.
Abstract
Objective
To investigate the response of glycated hemoglobin (HbA1c) to fasting plasma glucose (FPG) under different altitude conditions, and to evaluate its applicability in the diagnosis of type 2 diabetes mellitus (T2DM).
Methods
A cross-sectional study was conducted involving 410 patients with T2DM from Shanghai (0 m) and Xining (2261 m) in China between 2022 and 2024. Biochemical indices including FPG, HbA1c, and hemoglobin (Hb) were collected to analyze the impact of altitude on HbA1c values and their correlation with FPG. Statistical methods included Mann–Whitney U test, Pearson correlation analysis, analysis of covariance (ANCOVA), and effect size evaluation (Cohen's d, r).
Results
Patients in the high-altitude group had significantly higher levels of FPG (10.32 ± 3.82 mmol/L vs. 7.25 ± 1.74 mmol/L) and HbA1c (9.21 ± 2.71% vs. 8.06 ± 1.73%) (both p < 0.001), while the HbA1c-to-FPG ratio was significantly lower (0.961 vs. 1.148, p < 0.001), suggesting that under equivalent glycemic levels, HbA1c may underestimate true glycemic burden at high altitudes. Correlation analysis showed a moderate correlation between HbA1c and FPG in both groups (r = 0.532), although FPG variability was higher in the high-altitude group. ANCOVA revealed a significant effect of FPG stratification on HbA1c (p < 0.001) and a marginal interaction between altitude and FPG (p = 0.066). Additionally, patients with high Hb levels (> 160 g/L) had significantly lower HbA1c levels compared to those with lower Hb (6.3% vs. 7.0%, p < 0.001).
Conclusion
High-altitude environments may interfere with the accuracy of HbA1c by increasing hemoglobin levels, potentially leading to an underestimation of true glycemia. It is recommended that diabetes monitoring in plateau regions incorporate direct glycemic indicators such as FPG or CGM to optimize clinical decision-making and risk management.
Keywords:
glycated hemoglobin
altitude
type 2 diabetes mellitus
fasting plasma glucose
Electronic Supplementary Material
Below is the link to the electronic supplementary material
Introduction
Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by chronic hyperglycemia and has become a global public health challenge. According to the latest statistics published by the International Diabetes Federation (IDF) in 2025, there are 589 million adults aged 20–79 years living with diabetes worldwide, and this number is projected to rise to 853 million by 2050[1]. China bears the highest burden of diabetes globally, with 118 million individuals currently affected, accounting for 22% of the global diabetic population. T2DM is the predominant form, and the associated complications contribute significantly to healthcare expenditures and reduced quality of life[2].
Glycemic monitoring is central to diabetes management. Glycated hemoglobin (HbA1c), which reflects the average blood glucose over the preceding three months, has been recommended by both the World Health Organization (WHO) and the American Diabetes Association (ADA) as a key indicator for the diagnosis and long-term management of diabetes[3–5]. However, as an indirect marker of blood glucose, HbA1c can be influenced by various non-glycemic factors, such as red blood cell lifespan, hemoglobin concentration, and oxygen availability. These influences may impair the accuracy of HbA1c under certain physiological or geographical conditions. Although glycated albumin (GA) can reflect short-term glycemic changes and is unaffected by hypoxia, its limited clinical adoption hinders its widespread use as a substitute marker[6].
High-altitude regions (> 2000 m) are commonly characterized by hypoxic environments. To maintain effective oxygen delivery, the human body typically undergoes compensatory increases in red blood cell count and hemoglobin concentration. These hematologic changes may interfere with the formation and stability of HbA1c, which is generated through a non-enzymatic reaction between hemoglobin and glucose [7]. To date, studies on the impact of high altitude on HbA1c have yielded conflicting results. For example, Yao et al [8] reported a significant positive correlation between HbA1c and hemoglobin (Hb) levels among non-diabetic residents living at extremely high altitudes, indicating that higher Hb levels are associated with increased HbA1c. In contrast, McClain et al [9] found no significant difference in HbA1c levels between patients with T2DM living in highland and lowland regions of Turkey. Interestingly, Hessien [10] reported lower HbA1c levels in T2DM patients residing at moderate altitudes in Saudi Arabia compared to their lowland counterparts.
This study employed a cross-sectional design to compare the relationship between HbA1c and fasting plasma glucose (FPG) in patients with T2DM living in Shanghai (0 m) and Xining (2261 m) regions in China. We analyzed the HbA1c-to-FPG ratio, correlation, interaction effects, and the impact of hemoglobin levels on HbA1c to evaluate the applicability of HbA1c as a glycemic monitoring and diagnostic marker in high-altitude areas, aiming to provide empirical evidence for clinical decision-making in high-altitude regions.
Materials and Methods
Subjects and Ethical approval
A
This study adopted a cross-sectional design and included a total of 410 patients with diagnosed type 2 diabetes mellitus (T2DM) from both low-altitude (Shanghai) and high-altitude (Xining) regions in China between 2022 and 2024 (200 cases from the low-altitude group and 210 from the high-altitude group). A
Participants were recruited from the Third People’s Hospital of Xining, Hainan Tibetan Autonomous Prefecture Hospital in Qinghai Province, and the Department of Endocrinology at Huashan Hospital, Fudan University. Inclusion criteria were: (1) age ≥ 18 years; (2) diagnosis of T2DM according to WHO criteria; (3) stable blood glucose levels without acute infections, blood transfusions, or major surgery within the past three months. Exclusion criteria included: (1) severe anemia (Hb < 110 g/L); (2) diagnosed renal insufficiency, liver cirrhosis, or thyroid dysfunction; (3) gestational diabetes or other specific types of diabetes (e.g., MODY); (4) exposure to factors that may interfere with HbA1c measurement within the past three months, such as erythropoietin use or hyperbaric oxygen therapy.A
All participants provided written informed consent. A
The study protocol was approved by the ethics committee of the respective hospitals (approval number: 2022-M-12). A
A
All methods were performed in accordance with the relevant guidelines and regulations.Measurements and Laboratory Methods
Fasting Plasma Glucose (FPG): Measured using the glucose oxidase method. Glycated Hemoglobin (HbA1c): Measured by high-performance liquid chromatography (HPLC). Complete Blood Count (CBC): Red blood cells (RBC), hemoglobin (Hb), and other parameters were assessed using an automated hematology analyzer. Additional Indicators: Including 2-hour postprandial glucose (2hPG), glycated albumin (GA), and liver and renal function tests. Glycated Albumin (GA): Measured using the nitroblue tetrazolium (NBT) method developed by a domestic manufacturer, Sinothanks Biotech.
Statistical Analysis
All statistical analyses were performed using IBM SPSS Statistics version 27.0.1. The Shapiro–Wilk test was used to assess the normality of continuous variables. Based on distribution characteristics, appropriate statistical tests were selected. Normally distributed variables were compared using independent samples t-tests, while non-normally distributed variables were analyzed using the Mann–Whitney U test.
Pearson correlation coefficients were used to evaluate the linear relationship between HbA1c and FPG. To explore the independent effect of altitude on HbA1c while controlling for glycemic status, analysis of covariance (ANCOVA) was conducted, using FPG as a covariate. Estimated marginal means of HbA1c were compared across altitude groups, and interaction effects between altitude and FPG stratification were further analyzed.
Effect sizes were calculated to assess the practical significance of statistical differences. Cohen’s d was used to quantify the magnitude of group differences for continuous variables; η² represented the proportion of variance in HbA1c explained by each factor in ANCOVA; and r values were reported for nonparametric tests such as the Mann–Whitney U test.
Selected analysis results were visualized for clarity, including scatter plots of HbA1c versus FPG (Fig. 1), box plots comparing HbA1c across altitude groups (Fig. 2), interaction plots of altitude and FPG strata (Fig. 3), and the effect of hemoglobin levels on HbA1c (Fig. 4). Corresponding numerical results are detailed in Tables 1–2.
Results
As shown in Table 1, there were no statistically significant differences between the two groups in demographic characteristics such as age, sex, and BMI (all p > 0.05).
Regarding glycemic metabolism indicators, both fasting plasma glucose (FPG) and 2-hour postprandial glucose (2hPG) levels were significantly higher in the high-altitude group compared to the low-altitude group (FPG: 10.32 ± 3.82 vs. 7.25 ± 1.74 mmol/L; 2hPG: 15.31 ± 5.38 vs. 9.92 ± 2.77 mmol/L, both p < 0.001). HbA1c levels were also significantly elevated in the high-altitude group (9.21 ± 2.71% vs. 8.06 ± 1.73%, p < 0.001). However, the HbA1c-to-FPG ratio was notably lower in the high-altitude group (0.961 ± 0.327 vs. 1.148 ± 0.291, p < 0.001), suggesting an insufficient relative increase in HbA1c under equivalent glycemic conditions.
It is noteworthy that glycated albumin (GA) levels were significantly lower in the high-altitude group (2.47 ± 0.63% vs. 3.78 ± 1.52%, p < 0.001), which was inconsistent with the rising trend of HbA1c, implying potential influence from non-glycemic factors such as albumin metabolism.
In terms of hematologic parameters, the high-altitude group demonstrated typical adaptive responses to chronic hypoxia. Red blood cell (RBC) count (5.20 ± 0.68 vs. 4.75 ± 0.42 ×10¹²/L) and hemoglobin concentration (159.64 ± 19.66 vs. 141.77 ± 12.18 g/L) were significantly elevated (both p < 0.001), whereas lymphocyte and platelet counts were markedly decreased (p < 0.001). Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels were also slightly higher in the high-altitude group. Detailed comparisons are shown in Table 1.
Analysis of the entire sample revealed a moderate positive correlation between FPG and HbA1c (Pearson's r = 0.532, p < 0.001). Although the correlation coefficients between the high- and low-altitude groups were not significantly different (p = 0.16), the FPG standard deviation was significantly greater in the high-altitude group (SD = 3.83 vs. 1.74; Levene’s test p < 0.01), indicating greater glycemic variability, which may compromise the linear association between HbA1c and actual glucose levels. The scatter plot of HbA1c versus FPG is shown in Fig. 1.
Both FPG and HbA1c levels were significantly higher in the high-altitude group. Further group comparison showed that the mean FPG in the high-altitude group (10.32 ± 3.82 mmol/L) was 42% higher than that in the low-altitude group (7.25 ± 1.74 mmol/L, p < 0.001), with a large effect size (Cohen’s d = − 1.03). The mean HbA1c level (9.21 ± 2.71%) was 14% higher than in the low-altitude group (8.06 ± 1.73%, p < 0.001), corresponding to a medium effect size (Cohen’s d = − 0.51). Despite the elevation of HbA1c values in the high-altitude group, the HbA1c-to-FPG ratio significantly decreased (p < 0.001, d = 0.60), indicating a reduced responsiveness of HbA1c under comparable glycemic conditions and a potential underestimation of actual glycemic burden.
To further exclude the confounding effect of FPG, ANCOVA was performed with FPG as a covariate to assess the independent effect of altitude on HbA1c. The results showed a significant main effect of FPG stratification (F = 44.436, p < 0.001, η² = 0.180), indicating that HbA1c was strongly influenced by glycemic level. The main effect of altitude was not significant (F = 0.333, p = 0.564), but the interaction between altitude and FPG was marginally significant (F = 2.731, p = 0.066), suggesting that altitude may amplify HbA1c elevation in individuals with high FPG, whereas the impact is minimal in normoglycemic or mildly hyperglycemic individuals.
To investigate the influence of high-altitude erythrocytosis on HbA1c accuracy, samples were stratified based on hemoglobin levels. Results showed that individuals with Hb > 160 g/L had significantly lower HbA1c levels than those with Hb ≤ 160 g/L (6.3% vs. 7.0%, p < 0.001, r = 0.24), indicating a medium effect size. This trend was consistent across altitude groups, suggesting that increased Hb concentration may dilute the proportion of glycation and consequently underestimate HbA1c relative to actual glucose levels.
In summary, although absolute HbA1c levels increased in the high-altitude group, its responsiveness was reduced after adjusting for glucose levels, as evidenced by a lower HbA1c-to-FPG ratio and weakened correlation. This suggests that HbA1c may underestimate true glycemia in high-altitude populations. Such deviations may result from a combination of elevated RBC count, increased Hb concentration, and other environmental factors. In clinical practice, it is recommended that comprehensive assessment using FPG, CGM, or GA be employed in high-altitude regions to enhance the accuracy and individualization of glycemic management.
Discussion
This study evaluated the influence of different altitude environments on the relationship between glycated hemoglobin (HbA1c) and fasting plasma glucose (FPG) in patients with type 2 diabetes mellitus (T2DM). We found that although both FPG and HbA1c levels were significantly elevated in the high-altitude group (> 2000 m), the HbA1c-to-FPG ratio was significantly lower, suggesting that HbA1c may systematically underestimate the true glycemic burden under similar glycemic exposure. This finding aligns with studies conducted in other high-altitude populations, such as in Peru[11]. Furthermore, natural "hypoxia models" involving HIF pathway abnormalities, such as Chuvash polycythemia, have also observed concurrent decreases in Hb, blood glucose, and HbA1c levels, mechanistically supporting the chain of "hypoxia → hematological changes → HbA1c bias" [9].
Mechanistically, HbA1c is influenced by red blood cell (RBC) lifespan, hemoglobin concentration, and glycation kinetics. A shortened RBC lifespan or accelerated RBC turnover can significantly lower HbA1c levels even under identical average glucose levels[12][13][14]. Our stratified analysis showed that individuals with Hb > 160 g/L had relatively lower HbA1c, consistent with the inference that polycythemia leads to an underestimation of HbA1c. Notably, glycated albumin (GA) levels were significantly lower in the high-altitude group (2.47 ± 0.63% vs. 3.78 ± 1.52%, p < 0.001), a trend opposite to that of HbA1c. This divergence suggests that non-glycemic factors, potentially including altitude-induced alterations in albumin metabolism or glycation kinetics, may be systematically affecting the formation of glycated proteins. GA reflects short-term glycemic status over the past 2–3 weeks and, theoretically, is not affected by RBC lifespan or hemoglobin concentration. However, it may be influenced by albumin metabolism, inflammation, or oxidative stress[15][16].
On the other hand, a growing body of research shows systematic discordance between CGM-derived metrics (such as GMI and TIR) and laboratory HbA1c. Therefore, guidelines and reviews recommend reporting these metrics alongside HbA1c rather than equating them directly, to reduce misjudgment caused by bias in any single indicator [17][18]. The significantly lower GA levels we observed at high altitude, despite elevated HbA1c and FPG, provide compelling evidence for such a bias and underscore the need for a multi-marker approach.
Correlation analysis revealed a moderate positive correlation between FPG and HbA1c in the total sample (r = 0.532, p < 0.001). However, the standard deviation of FPG was significantly greater in the high-altitude group (3.83 vs. 1.74), which may compromise the linear association between HbA1c and actual glucose levels. ANCOVA further demonstrated a strong main effect of FPG stratification on HbA1c (F = 44.436, p < 0.001) and a marginally significant interaction between altitude and FPG (F = 2.731, p = 0.066). This marginal interaction suggests that altitude may amplify HbA1c elevation in individuals with high FPG, supporting the notion that the relationship between HbA1c and glucose is modulated by altitude.
In summary, although absolute HbA1c levels increased in the high-altitude group, its responsiveness was reduced after adjusting for glucose levels, as evidenced by a lower HbA1c-to-FPG ratio and weakened correlation. The concurrent finding of decreased GA reinforces the conclusion that high altitude introduces a systematic bias. This deviation may result from a combination of elevated RBC count, increased Hb concentration, and other environmental factors affecting glycation. Particularly noteworthy is that patients with hemoglobin levels > 160 g/L had significantly lower HbA1c levels (6.3% vs. 7.0%, p < 0.001), supporting the notion that increased hemoglobin concentration may dilute the proportion of glycation and consequently underestimate true glycemic levels.
When compared against guidelines and consensus statements, the American Diabetes Association Standards of Care in Diabetes-2024 explicitly states that HbA1c should not be used as the sole criterion for diagnosis in the presence of interfering conditions such as abnormal RBC lifespan or hemoglobin variants[19]. Numerous studies and international guidelines highlight that shortened RBC lifespan, hemoglobin variants, and related hematological abnormalities can interfere with HbA1c accuracy, thus limiting its utility in specific populations[20][21]. Our study, set against the specific backdrop of "high altitude → hypoxia → hematological adaptation," validates this view.
Considering that hematological adaptation to altitude varies across different ethnic populations[22][23], future research and clinical practice should consider stratification by ethnicity to avoid applying uniform HbA1c thresholds indiscriminately.
Based on these findings, we recommend that diabetes monitoring in high-altitude regions incorporate direct glycemic indicators such as FPG, GA, or CGM to improve the accuracy and individualization of glycemic assessment. Future research should focus on elucidating the mechanisms through which high altitude affects HbA1c formation and on developing multivariate correction models that incorporate altitude, hemoglobin concentration, and other variables to support precision diabetes care in plateau regions.
Conclusion
Based on data collected from 410 patients with type 2 diabetes mellitus, this study systematically assessed the influence of altitude on the relationship between glycated hemoglobin (HbA1c) and fasting plasma glucose (FPG) and evaluated the applicability of HbA1c as a glycemic monitoring indicator in high-altitude regions. The results demonstrated that although both HbA1c and FPG levels were significantly higher in the high-altitude group, the HbA1c-to-FPG ratio decreased markedly, and the stability of the HbA1c–FPG correlation weakened, suggesting that HbA1c may underestimate actual glycemia in high-altitude populations.
This deviation may be associated with elevated red blood cell count and hemoglobin concentration in high-altitude environments. Patients with hemoglobin levels > 160 g/L showed significantly lower HbA1c levels. Additionally, the observed decrease in GA (glycated albumin) in the high-altitude group provides further evidence of a systematic bias affecting glycated protein markers at high altitude. Covariance analysis revealed a marginal interaction between altitude and FPG, which supports the view that the impact of altitude on HbA1c is more pronounced under hyperglycemic conditions.
In conclusion, HbA1c results should be interpreted cautiously in high-altitude settings and should not be used as the sole indicator for glycemic monitoring or diagnosis. Clinical assessment is recommended to incorporate multiple indicators, including FPG, GA, or continuous glucose monitoring (CGM), to improve diagnostic accuracy and diabetes management. Future studies should focus on mechanistic investigations and the development of multivariable correction models to refine HbA1c interpretation in special populations and support precision diabetes care in high-altitude regions.
Funding:
This work was supported by grants from the Xining Science and Technology Project(no.2022-M-12) and the Noncommunicable Chronic Diseases-National Science and Technology Major Project (Grant No. 2024ZD0531805).
Data Availability
The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request.
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Author Contribution
S.G. and P.H. contributed conception and design of the study. S.G., N.R. and X.Z. recruited patients. X.Z., C.Y., L.T. and Y.L. collected the data. H.L.and S.M. analyzed the data.Y.Y. conducted the quality control and wrote the main manuscript textS.G, R.N.,and X.Z.revised the work critically for important intellectual content. All authors reviewed and approved the final manuscript
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Variable | 0 m (n = 200) | > 2000 m (n = 210) | P value |
|---|---|---|---|
Age (years) | 55.76 ± 9.16 | 55.72 ± 8.82 | 0.278 |
Gender (M/F) | 128/72 | 142/68 | 0.441 |
BMI (kg/m2) | 25.45 ± 3.59 | 25.48 ± 3.63 | 0.922 |
DM duration (years) | 6.72 ± 4.73 | 6.06 ± 5.55 | 0.195 |
FPG (mmol/L) | 7.25 ± 1.74 | 10.32 ± 3.83 | < 0.001 |
2hPG (mmol/L) | 9.92 ± 2.77 | 15.31 ± 5.38 | < 0.001 |
HbA1c (%) | 8.06 ± 1.73 | 9.21 ± 2.71 | < 0.001 |
GA (%) | 3.78 ± 1.52 | 2.47 ± 0.63 | < 0.001 |
Albumin (g/L) | 42.39 ± 2.75 | 42.42 ± 4.62 | 0.928 |
WBC (*10^9/L) | 6.48 ± 1.39 | 6.26 ± 1.68 | 0.148 |
Neutrophils (*10^9/L) | 3.81 ± 1.09 | 4.01 ± 1.41 | 0.106 |
Lymphocytes (*10^9/L) | 2.17 ± 0.65 | 1.79 ± 0.61 | < 0.001 |
RBC (*10^12/L) | 4.75 ± 0.42 | 5.20 ± 0.68 | < 0.001 |
Hemoglobin (g/L) | 141.77 ± 12.18 | 159.64 ± 19.66 | < 0.001 |
Platelets (*10^9/L) | 218.87 ± 52.45 | 181.46 ± 59.69 | < 0.001 |
ALT (U/L) | 25.35 ± 9.72 | 34.14 ± 25.92 | < 0.001 |
AST (U/L) | 19.82 ± 5.99 | 22.84 ± 16.65 | 0.016 |
Urea (mmol/L) | 5.74 ± 1.43 | 5.95 ± 1.84 | 0.205 |
Creatinine (umol/L) | 67.12 ± 15.38 | 66.02 ± 15.66 | 0.477 |
| Note: Values are presented as mean ± SD or counts unless otherwise specified. | |||
Fig. 1
Scatter plot showing the correlation between HbA1c and FPG.
Fig. 2
Boxplot of HbA1c levels by altitude group.
Note
High-altitude group shows significantly higher median HbA1c (p < 0.001).
Fig. 3
Interaction plot of altitude and FPG stratification on estimated marginal means of HbA1c.
Factor | F | P | η² | 显著性结论 |
|---|---|---|---|---|
Altitude group | 0.333 | 0.564 | 0.001 | Main effect of altitude: not significant |
FPG group | 44.436 | < 0.001 | 0.180 | Highly significant (higher FPG associated with higher HbA1c) |
FPG group × Altitude group | 2.731 | 0.066 | 0.013 | Marginally significant interaction between altitude and FPG stratification |
Fig. 4
Effect of hemoglobin concentration on HbA1c levels in different altitude groups.
