Seasonal variation of immune thrombocytopenic purpura related hospitalizations among adults in the USA: analysis of the nationwide inpatient sample database

Prior studies in the USA and elsewhere have suggested a distinct seasonal pattern in the incidence of immune thrombocytopenic purpura (ITP) among children [Kuhne et al. 2001; Moussalem and Yassine, 2003; Watts, 2004; Zeller et al. 2005; Yong et al. 2010]. However, it remains unknown if such a seasonal trend exists among adults with ITP-related hospitalizations.

We utilized the Nationwide Inpatient Sample (NIS) database to study the seasonal pattern of ITP-related hospitalizations among adults in the US. NIS is the largest inpatient database in the USA and is sponsored by the Agency for Healthcare Research and Quality. International Classification of Diseases ninth revision, Clinical Modification code 287.31 was used to identify all patients discharged with a principal diagnosis of ITP between 1 January 2005 and 31 December 2011. We stratified each hospitalization with a primary diagnosis of ITP by the month of presentation. Walter and Elwood modification of the Edwards test was used to determine the monthly variation in the frequency of hospitalizations. Statistical analysis was conducted using STATA-MP 13.0 (StataCorp LP, College Station, TX, USA).

ITP accounted for a total of 69,981 hospitalizations between 2005 and 2011. No significant monthly variation in ITP-related hospitalizations was seen during the study period (Figure 1). There was a nonsignificant trend towards peak hospitalization during the month of September (peak/low ratio of 1.01, 95% confidence interval 1.0–1.03).

Figure 1.

Monthly frequency distribution of immune thrombocytopenic purpura (ITP)-related hospitalizations in the USA based on the Nationwide Inpatient Sample database 2005–2011.

Our study shows a lack of a seasonal trend among ITP-related hospitalizations among adults in the USA. This is in contrast to childhood ITP, for which a distinct seasonal trend has been shown [Kuhne et al. 2001; Moussalem and Yassine, 2003; Zeller et al. 2005; Yong et al. 2010]. The seasonal pattern among children with ITP likely reflects the seasonality of viral infections such as influenza, which is responsible for the majority of cases of ITP in children [Yong et al. 2010]. Unlike in children, ITP in adults has an insidious onset and a chronic course with no preceding viral illness or other infection [Liu et al. 2013].

The large sample size and inclusion of different regions of the USA are the main strengths of our study. However, our study has several limitations. It was a retrospective study based on discharge-level patient data with limited clinical information. NIS is a deidentified administrative database and is prone to errors in coding of diagnoses that cannot be individually verified.