Stability of serum, plasma and urine osmolality in different storage conditions: Relevance of temperature and centrifugation.

BACKGROUND: Osmolality reflects the concentration of all dissolved particles in a body fluid, and its measurement is routinely performed in clinical laboratories for the differential diagnosis of disorders related with the hydrolytic balance regulation, the renal function and in small-molecule poisonings. The aim of the study was to assess the stability of serum, plasma and urine osmolality through time and under different common storage conditions, including delayed centrifugation.

METHODS: Blood and urine samples were collected, and classified into different groups according to several preanalytical variables: serum or plasma lithium-heparin tubes; spun or unspun; stored at room temperature (RT), at 4°C or frozen at -21°C. Aliquots from each group were assayed over time, for up to 14days. Statistical differences were based on three different international performance criteria.

RESULTS: Whole blood stability was higher in the presence of anticoagulant. Serum osmolality was stable for 2days at RT and 8days at 4°C, while plasma was less stable when refrigerated. Urine stability was 5days at RT, 4days at 4°C and >14days when frozen.

DISCUSSION: Osmolality may be of great interest for the management of several conditions, such as in case of a delay in the clinical suspicion, or in case of problems in sample collection or processing. The ability to obtain reliable results for samples kept up to 14days also offers the possibility to retrospectively assess baseline values for patients which may require it.