An inter-laboratory proficiency trial was undertaken as a prelude to a survey of Australian drinking source waters for Cryptosporidium and Giardia oo/cysts. Specific aims included measuring the recovery of protozoan control material from representative source waters, identifying laboratory errors and assessing the utility of pre-stained reference oo/cysts. Five experienced laboratories were provided with coded vials containing oo/cysts or blanks for spiking 10 L water samples (11 replicate environmental, 5 filtered) or to be analysed directly (16 Count Control vials). Each laboratory reported both their standard counts and details of how each oo/cyst image was interpreted and confirmed. A sixth laboratory provided flow cytometry prepared seed material and reference analyses. Average recoveries for 10 L samples ranged from 0-67% and 0-72%, for environmental and filtered tap water respectively, depending on laboratory and sample type. The best performing laboratory obtained high recoveries, ranging from 45% to 66% for tap water and 63% to 81% for environmental water. Nonetheless, recovery from tap water and environmental sample analysed simultaneously by the same laboratory could differ markedly (1% v. 48% in one case). Poor recovery and false positives were encountered despite the experience of the participants. Inconsistencies in reporting were introduced by the different procedures for confirming oocysts and cysts. The trial showed that obtaining reliable estimates of environmental protozoan loads is still a concern for water managers.
- quality control
- © IWA Publishing 2002