Kits contain flexible plastic strips to which filter paper containing nutrients are attached. Aerobic bacteria form red spots on the test strip. Yeast and mold form black spots on the test strip. Use the number of spots along with the degree of background reddening for the aerobic bacteria to get a measurement of microbial growth.

For the yeast and mold, use the number of spots along with the degree of graying of the background to get a measurement of yeast and mold growth. After an incubation period, compare the strip to a color chart on the label for an estimated bacterial count.