Success in the toxoplasma dye test.

OBJECTIVES: To compare the success rate of the toxoplasma dye test using different accessory factors (human serum as a source of complement) and different batches of tachyzoites produced in vivo and in vitro.

METHODS: Twenty-five accessory factors were used in the dye test to assess both types of tachyzoite. Batches of tachyzoites were produced in vivo (n = 49) and in vitro (n = 23) and their performance assessed against panels of accessory factors. Performance was recorded as success or failure (incorrect results, total killing or no killing).

RESULTS: With in vivo tachyzoites 21/25 accessory factors were successful in P > or = 1 dye test runs, whereas with in vitro tachyzoites all 25 were successful. One or more failure was recorded for 19/25 and 12/25 accessory factors using in vivo and in vitro tachyzoites, respectively (P < 0.05). The number of successful dye test runs was less for in vivo (92/141, 65%) than in vitro (140/163, 86%) tachyzoites (P < 0.001). This was due to a higher success rate in citrated accessory factors used for in vitro tachyzoites compared to the corresponding uncitrated accessory factors used for in vivo tachyzoites (P < 0.001). Success in the dye test was recorded for 48/49 and 23/23 batches of in vivo and in vitro tachyzoites, respectively. The number of successful dye test runs was lower with in vivo (156/234, 67%) than in vitro (116/142, 82%) tachyzoites (P < 0.01).

CONCLUSIONS: Success in the dye test may be due to the accessory factor, tachyzoites, or a combination of both. Problems due to the accessory factor can be minimized by careful quality control or use of modification procedures. Tachyzoites produced in vitro may also increase success in the dye test. Careful selection of accessory factor/tachyzoite combination makes it possible to use the dye test in a district general hospital.