SOURCES OF ERROR IN COUNTS 



17 



would ordinarily give an official plate 

 count of less than 10,000 per cc. The 

 grading of milk of this type must be 

 done with especial care as persons 

 inexperienced with microscopic work 

 have been found readily to confuse 

 extraneous objects with bacteria, in 

 milk containing very few organisms. 

 Where the number is less than 30 per 

 60 fields (average of less than yi a 

 bacterium per field), it may be assumed 

 that the ofiicial plate count will be less 

 than 60,000 per cc. Where the num- 

 ber is less than 100 per 60 fields (aver- 

 age of less than 1 ^ bacteria per 

 field), it may be assumed that the 

 official plate count will be less than 

 200,000 per cc. Where the number is 

 less than 1,000 per 60 fields (average 

 of less than 16 % bacteria per field), 

 it may be assumed that the official plate 

 count will be less than one to two million. 

 The standards given are computed 

 (with the exception of the poorest 

 grades) on the assumption that the 

 official plate count will normally 

 average 1/5 of the total number of 

 individual bacteria present. As many 

 cases will be found which diverge 

 markedly from the average, it is self 

 evident that this average represents 

 only an approximation to the real 

 conditions in any specific case, so that 

 in some cases the microscopic grading 

 will be more severe than that based 

 on the plate counts, and vice versa. 

 There is still a lack of sufficient data 

 from which to judge fairly which 

 system of grading is the more accurate. 

 The indications are, however, that 

 where the work is done with equal 

 skill and care, and the allowances 

 indicated are made, a reasonably close 

 agreement in grade will be secured^". 

 This fact is highly reassuring as to the 

 general accuracy of both systems of 

 grading. 



In the routine grading of milk by the 

 microscopic method it is not expected 

 that exact counts will be made. A 

 high grade milk will show field after 

 field of the microscope in which no 

 bacteria are seen, while a poor grade 

 of milk will show numerous bacteria 

 in every field examined. It is only 

 where the number of bacteria present 

 is close to the border line between 

 grades that counts need to be made. 

 The examination, however, must be 

 sufficiently thorough to make sure of 

 the grade as specified above. 



In order to ensure careful work in 

 grading, it is required that laboratories 

 conforming to standard procedure shall 

 preserve microscopic preparations until 

 a reasonable period has elapsed after 

 the reports are rendered to the person 

 or persons whose milk has been exam- 

 ined. It is an excellent custom 

 occasionally to have the grading done 

 by one analyst repeated by a second 

 analyst, particularly in those cases 

 where punitive actions are to be 

 based on the reports made. 



Common Sources of Error in Count. 



Routine microscopic counts, like 

 all bacterial counts, are to be 

 regarded as estimates of numbers only. 

 They cannot be made with absolute 

 accuracy even with the most careful 

 technique. Errors will arise from in- 

 accuracies in measurement of the 

 minute quantities of milk examined at 

 any one time, from faulty staining or 

 preparation of slides, from mistakes in 

 observation and the like. These limi- 

 tations, while important, are not 

 difficult to overcome in sufficient 

 measure to make microscopic grad- 

 ing a satisfactory method of controlling 

 the quality of unpasteurized milk. 

 As it is only in this way that counts 

 of the bacteria themselves can be 



