To reduce fading of the DAPI stain and preserve the filters on the microscope, always close the shutter between the mercury bulb and the filters when not viewing the oocyst.
Some analysts have noted that DAPI staining of Cryptosporidium usually is DAPI negative or DAPI positive with nuclei stained; DAPI positive with intense blue internal staining is observed infrequently.
Occasionally, the intense fluorescent staining of DAPI may appear white.
When viewed with DAPI filters, the FITC fluorescing rim will be nonexistent or appear light green. Other colors are atypical.
Experienced analysts find characterization at 1000X is necessary, even though a minimum of 400X is required by Method 1623.
No significant correlation between viability and DAPI staining has been scientifically established.