Moisture content of Hemoccult slides influences test sensitivity

Mass screening programmes in Europe and in the USA have shown that a 15-18% decrease in mortality rate from colo-rectal cancer can be achieved with biannual testing. In a 10 year screening programme in the Isère region (France), we have noticed that the positivity percentage of Hemoccult II changes significantly with seasons: it varies from 1.61% in summer to 2.80% in winter, while intermediate values occur in spring (1.69%) and autumn (2.50%). During transport in the mail, Hemoccult slides are stored in unheated rooms, without air-conditioning, and are thus being exposed to outdoor conditions of temperature and humidity. In this paper, we show that the moisture content of Hemoccult slides influences test sensitivity.

In an in-vitro experiment, we smeared slides from the same batch all with the same blood-spiked stool sample. Positivity rate varies from 58%, when slides contain 0.46% moisture, to 84% when they contain 10.90% moisture (P = 0.0066). Positivity rates are not different in rehydrated and in saturated Hemoccult slides.

Data collected from the Grenoble meteorological centre were compared to positivity percentages. Regression studies show that moisture deficit in the air is strongly correlated (r = 0.794, P < 0.01) to positivity percentage and explains most of its seasonal variations.

These results would suggest that Hemoccult slides should be allowed to equilibrate for 24 h at the same level of moisture in all seasons at least in regions where large temperature variations are observed between winter and summer, and between day and night. Indeed, positivity percentages are probably too high in winter, and too low in summer.