Comparison of sequential, random, and hemacytometer methods for counting Cryptosporidium oocysts

In animal and clinical studies of Cryptosporidium infection, counting Cryptosporidium oocysts in collected fecal samples is often critical to interpretation of results and validity of statistical analyses. In this experiment, 3 methods of counting oocysts derived from feces of infected mice were compared. These included sequential counting of oocysts in 25 adjacent microscopic fields, random counting of oocysts in 25 scattered microscopic fields, and hemacytometer counting of oocysts using standard procedures. Numerical results are expressed as the numbers of oocysts per microscopic field for sequential and random counting methods or per μl for the hemacytometer method. Our results showed that the numbers of oocysts adjusted for volume and weight, or for volume alone, were similar for all 3 techniques. Linear relationships between the 3 methods allow use of linear regression to convert the number of oocysts counted by 1 method into those counted by another. This suggests that comparisons between different experiments in which Cryptosporidium oocysts are counted using any of these methods are both possible and valid.