The Incidence of Aplastic Anemia in Thailand: Experiences from a Nationwide Registry

Introduction: Aplastic anemia (AA) has been reported to be 2-3 fold more frequent in Asia than in Western countries. Among the Asian population, its incidence varies geographically, ranging from 3.0 to 8.2 per million. A population-based case-control study conducted in Thailand from 1989 to1994 indicated that the annual incidence of AA was 3.0-5.0 per million. The incidence of AA may vary also among populations and study time frame therefore we conducted a nationwide aplastic anemia registry in Thailand. This prospective study determined the present incidence of AA according to each part of Thailand and identified demographic characteristics including environmental exposure.

Method: Newly diagnosed adult patients (≥15 years old) with AA were enrolled during the period from August 2014 to July 2016 in 32 participating hospitals across Thailand. Eligible patients were Thai citizens, newly diagnosed with hypocellular marrow plus at least two of the following hematologic parameters: anemia with corrected reticulocyte count ≤1%, absolute neutrophil count (ANC) ≤1.5x10⁹/L, platelet count ≤50x10⁹/L. Severity was classified as non-severe AA (non-SAA), severe AA (SAA) and very severe AA (vSAA), based on the modified Camitta criteria except corrected reticulocyte count ≤1%. Baseline characteristics and demographic data including viral hepatitis infection and environmental exposure were collected. Incidence of AA was calculated based on catchment area in six regions countrywide.

Results: A total of 346 patients were recruited in the registry. The patients' characteristics are shown in Table 1. One-hundred seventy nine patients (51.7%) were male, the median age was 59 years old (range 15 - 93). Among 346 patients, 59 patients (17.0%), 240 patients (69.4%) and 47 patients (13.6%) were non-SAA, SAA and vSAA, respectively. The overall incidence was 4.6 per million per year (5.0 in males and 4.3 in females). The incidence was higher with increasing age and the highest incidence was at 80-89 years (14.5 per million per year) (Figure 1). The highest incidence (6.3 per million per year) was observed in eastern region where industrial factories were populated, followed by northeastern (5.5) northern (3.8), western (3.8), southern (2.7), and central (2.7) regions (Figure 2). In comparison to a previous study published in 1999, the annual incidences per million in central (2.7 vs. 3.9) and southern (2.7 vs. 3.0) in this study were lower, whereas the incidence in northeastern region (5.5 vs. 5.0) was higher (Table 2). Seventy-four of 346 patients (21.4%) reported the environmental exposure within 6 months before diagnosis. Agricultural pesticides were the most commonly observed (17.3% of patients). This reported exposure was highest in patients who live in northeastern region (10.1%). In contrast, only 1.4% of eastern patients reported exposure to agricultural pesticides. Immunosuppressive therapy (IST) is more frequently used in SAA patients (62.8%) than vSAA (44.7%) whereas anabolic steroids are common used in non-SAA patients (83.0%). The main reasons for not receiving IST in SAA/vSAA were older age, poor health status and patient refusal.

Conclusion: This is the first nationwide study of incidence of aplastic anemia in Thailand. The results demonstrate the high incidences of AA throughout Thailand, supporting a previous study. The observed incidence was highest in the eastern region where industrial factories are common. Whether environmental or occupational exposure is the risk factors for AA requires further investigation.

This study was funded by sanofi-aventis (Thailand) Ltd.

View largeDownload slide

View largeDownload slide

Close modal