Microscopic counts of mycobacteria. Conversion factors for the pinhead method.

O)1e merit of the pinhead method ( t ) of transfering mierosamples of bacteria to glass slides is the rapidity with which large numbers of samples can be placed on each slide. The sys tem assures identical stainin g, diffcrentiation , and rinsing of eight samples p er slide. Nakayama et aT. (~) reported th e standard deviation between volum es per pinhead to be 7.4 p er cent and concluded that the method is more practical and convenient than other available procedures. With precautions to avoid atypical or defective films (see 1, PnOCEDUnE), the degree of reproducibility is further improved. A unique feature is that conversion factors can readily be calculated for any microscope, thus eliminating all the problems of calibra tion. The method of countin g and the deriva tion of conversion factors have already been described (1). With any given microscope, the number of fi elds observed per diameter of some fi lms may be one less (or one more) than the average. For example, with a system and a microscope that usually yields 30 fields/ film diameter, the number observed will sometimes be 31 or 29 fields. If one regards the average diameter as 30 fields and uses the corresponding conversion factor, the estimates are somewhat less accurate than when a conversion tabl e is used that shows the fac tors for 29, 30 and 31