Freezing of homogenized sputum samples for intermittent storage

Among the reasons that restrict the application of sputum induction in outpatient settings is the need for processing of samples within 2 h after induction.

The aim of our study was to assess whether freezing is suitable for intermediate storage of sputum samples before processing.

We compared differential cell counts between two sputum aliquots derived from the same sample. One aliquot was processed within 2 h after production and one, after it had been frozen under addition of dimethyl-sulfoxid (DMSO) and stored up to 10 days at -20 degrees C. Thirty-five samples were frozen immediately prior to preparation of cytospins, and 10 samples were frozen at an even earlier stage, directly after homogenization.

In both sets of experiments we observed a significant relationship between frozen and native samples regarding macrophages, neutrophils and eosinophils, as indicated by respective intraclass correlation coefficients of 0.96, 0.96, and 0.93 in the first, and of 0.92, 0.96 and 0.77 in the second experiments.

Our results indicate that the freezing of sputum samples at different stages of processing does not alter sputum morphology to an extent that affects the results of differential cell counts.