[Winter surveillance of cold exposure effects on health among the homeless population in the Paris area: data from the Coordinated Health Surveillance of Emergency Department network (Organisation de la surveillance coordonnée des urgences [Oscour(®)])].

Background: A program for helping homeless individuals in winter is implemented from November 1(st) to March 31(st) each year in France. Its aim is to prevent morbidity and mortality in this population during cold spells and periods of severe cold. A health surveillance system of the homeless population in the Paris area has been proposed to evaluate the effectiveness of the program and to alert decision-makers if an unusual increase in cold-weather effects is observed. The goal of this study was the creation of an indicator for the proposed surveillance system based on emergency department activity in the Paris area (Oscour(®) Network - Organisation de la surveillance coordonnée des urgences).

Methods: The winter 2007-2008 computer medical files of 11 emergency departments in the Paris area were examined to confirm diagnosis and ascertain patient-homelessness for each patient visit which was selected from the Oscour(®) database by the patient chief-complaint or diagnosis code referring to hypothermia or frostbites. The proposed indicator is based on the maximization of three criteria: the positive predictive value, the proportion of people identified as being homeless and the number of emergency department visits. A Shewhart control chart was applied to the indicator for the four winters between 2005 and 2009 in the Paris area. Values beyond the statistical threshold would indicate a need for an adjustment to the program strategy.

Results: Two hundred and sixteen medical files were analyzed. An indicator was created, "number of emergency department visits of 15 to 69-years-old persons with chief-complaint or diagnosis code referring to hypothermia". It had a positive predictive value estimated near 85 % and identified 61.7 % people as being homeless. In the winter of 2008-2009, the statistical threshold was reached in December during the first cold spell, and again at the beginning of January during a period of severe cold.

Conclusion: Our results support the use of this health indicator, alongside social indicators, for optimizing the strategy for helping the homeless population during winter.