Determination of Analyst and Method Variabilities Associated with Enumeration of Giardia and Cryptosporidium in Spiked Quality Control Samples

摘要

Three studies were conducted to: 1) assess variability between analysts countingrnprepared suspensions of Giardia and Cryptosporidium on well slides andrnhemacytometers, 2) assess counting method variability (well slides and hemacytometerrnchamber counts) from batch beaker aliquots containing approximately 100 Giardia cystsrnand 100 Cryptosporidium oocysts per 10 μL, and 3) assess counting method variabilityrn(well slides and chamber counts) from vial aliquots prepared from the batch beakers. Inrnthe first study, each analyst (n=4) counted both Giardia and Cryptosporidium on thernsame well slide 7 times at 200X magnification using the same microscope . The greatestrnaverage percent difference between analysts was 6.8% for Giardia and 9.0% forrnCryptosporidium. This experiment was repeated using a hemacytometer. Each analystrn(n=3) counted the same four outer 1mm areas of the same hemacytometer load 3 times. 2rnFor this study, the greatest percent difference between analysts was 3.3% for Giardia andrn15.2% for Cryptosporidium. In the case of Cryptosporidium, this greater difference wasrna function of a total count of 28-33 organisms per 4mm (lower counts resulted in higher 2rnvariability).rnIn the second study, where aliquots were assayed from a common batch beaker,rnanalysis of variance (ANOVA) tests showed that there were no significant differences inrnmean organism, for either organism, for counts among the 3 analysts. However, there werernstatistical differences between the counting methods. The results for both Giardia andrnCryptosporidium were identical. In trial 1 the chamber count gave higher counts; in trialrn2 there was no difference; and in trial 3, the well slide method gave higher counts. Thernvariability among analysts ranged from 2.1% to 9.2 %.