Seasonal and regional variation of thrombotic thrombocytopenic purpura in the <scp>U</scp>nited <scp>S</scp>tates: Insights from the <scp>N</scp>ationwide <scp>I</scp>npatient <scp>S</scp>ample
Authors: Smith Giri, Ranjan Pathak, Madan Raj Aryal, Paras Karmacharya, Vijaya Raj Bhatt, Mike G. Martin
Journal: American Journal of Hematology (2014)

Previous studies have reported conflicting results regarding the seasonal variation in the incidence of thrombotic thrombocytopenic purpura (TTP) in the United States 1-5. These studies are single institutional coming from different regions of the US and are limited by sample size. To overcome this limitation, we utilized a large national database to determine the seasonal trend of TTP-related hospitalizations in the US. We used the Nationwide Inpatient Sample (NIS) database to identify patients with TTP discharged between January 1, 2009 and December 31, 2011 using international classification of diseases, ninth revision, clinical Modification (ICD-9-CM) code 446.6. NIS is the largest publicly available all-payer inpatient care database in the United States and is sponsored by the agency for healthcare research and quality as a part of healthcare cost and utilization project 6. We included only those patients who received therapeutic plasmapheresis (ICD-9-CM procedure code 99.71) during hospitalization to capture active cases of TTP and improve coding accuracy. We derived the monthly distribution of TTP admissions and further stratified the study cohort into four regions (Northeast, Midwest, West, and South). Z test was used to determine if statistically significant differences existed in the monthly distribution of TTP cases. Statistical analysis was done using statistical package for social sciences. An estimated 6,360 TTP-related hospitalizations occurred in the US between 2009 and 2011. The majority of patients received care in the Southern region (35%, N = 2,211). The proportion of admissions from the rest of the regions was as follows: Northeast: 20%, Midwest: 30%, and west 15%. A significant monthly variation in the overall TTP-related hospitalizations was not observed. However, after stratifying the cases based on the regions, a significant monthly variation was noted in the Midwestern region. In this region with a total of 1,910 cases of TTP, the peak incidence observed during July was significantly higher than the lowest incidence noted in November (12.1% vs. 4.8%, P < 0.001) (Fig. 1). However, there were no statistically significant monthly variation of TTP admissions in the Southern, Northeastern, and Western regions. Monthly variation of the overall TTP related hospitalizations in the United States. Our study demonstrated a lack of seasonal variation among overall TTP related hospitalizations in the US; however, a significant variation was seen in the midwestern region. In the United States, five studies have assessed the seasonality of TTP cases, at various centers and regions in the US, which have shown conflicting results 1-5. The difference in seasonal trends of TTP among these institutions may be related to the geographic, environmental, and population differences among various regions of the US 4, 5. Only the study from the Midwest region had reported an increased number of TTP cases in summer and autumn, concomitant with our findings 2. Also, certain environmental factors such as infections and vitamin D levels have a marked seasonal fluctuance and may contribute to these variations 4. The large sample size and inclusion of different regions of the US are the main strengths of our study. However, the national inpatient sample is a deidentified administrative data prone to errors in ICD-9 CM coding, which cannot be individually verified. We were also unable to separately analyze patients with acquired versus congenital TTP as well as the initial versus recurrent episode of TTP since these are not captured by NIS database separately. Smith Giri,1* Ranjan Pathak,2 Madan Raj Aryal,2 Paras Karmacharya,2Vijaya Raj Bhatt,3 and Mike G. Martin1 1Department of Medicine, University of Tennessee Health Science Center, Memphis, Tennessee; 2 Department of Internal Medicine, Reading Health System, West Reading, Pennsylvania; 3 Department of Internal Medicine, Division of Hematology and Oncology, University of Nebraska Medical Center; Omaha, Nebraska