Final answer:
Black dots on an instrument disinfected with ortho-phthaldehyde suggest improper cleaning, as these dots can be stains from residual organic material. A positive in-use test implies microbial contamination of the disinfectant solution.
Step-by-step explanation:
When an improperly cleaned instrument that has been disinfected in ortho-phthaldehyde shows black dots, it typically indicates that the instrument has not been cleaned properly. These black dots are likely to be deposits or stains from residual organic material that was not completely removed during the cleaning process. Ortho-phthaldehyde is a high-level disinfectant used in healthcare settings for sterilizing medical instruments, particularly those that are heat-sensitive, but its effectiveness depends on prior thorough cleaning of the instruments. A positive in-use test for chemical disinfectants like ortho-phthaldehyde would indicate that the solution is contaminated with microbes, hence its efficacy in disinfecting properly cleaned instruments may be compromised.