COUNTING OF SOMATIC-CELLS IN MILK - STUDI ES ON THE MEASUREMENT USINGFLOW-CYTOMETRY (SOMACOUNT) AND COMPARISON WITH THE MEASURING RESULTS OBTAINED BY MEANS OF THE FLUORESCENCE OPTICAL-SYSTEM (FOSSOMATIC-360)

For counting somatic cells in milk (''cell counting'') as part of the EC/EU- and national regulations standardization is indispensable; this includes, in particular, the comparability of methods. In the present study flow cytometry (Somacount) is compared with the fluorescence op tical system (Fossomatic 360), as well as with optical count (Breed) a s reference method, the aim being to establish, to what extent the new method yields comparable results. Precision was calculated from multi ple counts. In the range below 500 (in 1000 cells/ml) the s.d. of repe ated countings does not exceed 10, the variation coefficient in the ra nge between 250 and 1.000 is 2 %, and beyond this range 1.6 %. After p retreatment using the preservatives potassium dichromate and Bronopol the samples keep for more than 48 h. With a limitation in time the sam e results in terms of shelf life are obtained when sodium azide and bo ric acid are used. The shelf life of the ''raw milk method'' (cold sto rage at 6-degrees-C) does not lead to temporal stability. The measurin g results between preservation methods are comaparable, when potassium dichromate, boric acid or Bronopol were used. With the ''raw milk met hod'' slightly lower values are obtained. Preservation with sodium azi de leads to markedly lower values. This is in agreement with the exper ience gained with other instruments. In the normal measuring range the carry over error of the apparatus from sample to sample is 1-2 %, and , hence, of no importance. No problems were encountered with the tempo ral stability of the measuring method for 4 days using potassium dichr omate. With plus/minus 2-3 % relative differences (means of repeated c ountings) are within the range of general accuracy. Using 407 quarter foremilk samples Somacount and Fossomatic were compared; with untransf ormed scale a correlation coefficient of 0.998 was found and after log transformation 0.917. In units of the log scale the residual s.d. is 0.06 in the range between 40 (in 1000 cells/ml) and 2.500, which corre sponds to a factor of 1.14. This means, the relative error is 14 %. As to the counting level a 1:1 agreement is found. The optical method's missing accuracy is disadvantageous for comparisons. Considered all in all, however, there are no substantial differences in the counting le vel compared with the Somacount. A first international collaborative t rial involving 17 laboratories with altogether 26 sections was perform ed according to the IDF standard 148:1991. In this intercomparison tri al 10 milks samples in the range between 129 and 1.086 (in 1000 cells/ ml) were used. A first evaluation showed a satisfactory repeatability (r = 26-103). Reproducibility was poor (R = 62-541) for the 10 milks ( = cell levels). This was, however, solely due to the inexperience of o ne laboratory only. After exclusion of this laboratory good intercompa rison test results were obtained (r = 26-95, R = 14-89). The IDF-targe ts for cv(r) and cv(R) are clearly observed. With a cell level of 428 cv(r) is 4.1 % (target = 4-5 %) and cv(R) 8.1% (target = 10-12%). Henc e, the Somacount instrument itself and the participating laboratories fulfil, after adequate training, the targets of the IDF standard 148:1 998. Good Laboratory Practice (calibration of apparatus) is supported by reference materials, which are treated using thermo-chemical method s and keep for several months (''Kiel standards''). Microscopically co ntrolled numbers of cells are available in biological matrix for contr olling the laboratory equipment. There exist no systematic differences between the values counted in the standard samples prior to (raw milk ) and after treatment.