The determination of cross-reacting organisms is one of the most important skills of an analyst. It takes many years of experience examining many different water matrices to be proficient. It is always helpful to discuss the characteristics of the organisms with another analyst (e.g. looking through the same microscope or sharing photographs or video at conferences). If another analyst is not available, examining various photographs helps.
Features indicating a cross-reacting organism:
- Atypical fluorescence – too wide, irregular, incorrect intensity, incorrect color
- Atypical DAPI – too many nuclei, incorrect size, wrong color
- Atypical DIC – wall too wide, incorrect internal morphology, presence of additional structures
Some field samples are very challenging and will take time to complete. The goal of every analyst is:
- Accurate and precise enumeration
- Accurate and consistent identification
- Learning from each sample examined

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