Peripheral blood monocyte counting: towards a new reference method.

Flow cytometric enumeration of monocytes stained with fluorescence-labelled monoclonal antibodies has been proposed as a possible reference method for monocyte counting. We compared precision and accuracy of monocyte counting of the Coulter STKS, the Cobas Argos 5 Diff, the 800-cell manual differential, and the Coulter Epics Profile II flow cytometer using double-staining with fluorescence-labelled monoclonal antibodies (CD45-FITC and CD14-PE). Precision: STKS, Argos and Profile II achieved a precision analogous to a 3423-, 1298-, and 11089-cell differential, respectively, confirming the superiority of automated methods. Accuracy (136 normal and abnormal samples): Correlation of automated methods with the manual differential was good (STKS: r = 0.934, Argos 5 Diff: r = 0.808, Profile II: r = 0.924; Spearman's rank correlation coefficient). The mean relative STKS monocyte result was 0.52 +/- 1.63% (mean +/- SD) higher than the manual differential, whereas the Argos 5 Diff results were 1.22 +/- 2.51% lower (p < 0.001). Profile II results showed a small bias against the manual differential (-0.18 +/- 1.44%, p < 0.05). Analysing 135 healthy adult subjects on the Profile II, males were found to have a higher mean monocyte count (relative count: 6.95 +/- 1.43% vs. 5.86 +/- 0.98%; absolute count: 0.48 +/- 0.15 x 10(9)/l vs. 0.39 +/- 0.11 x 10(9)/l, p < 0.001) and a higher and wider normal range than females (relative count: 4.97 to 9.78% vs. 4.26 to 7.81%, absolute count: 0.30 to 0.84 x 10(9)/l vs. 0.25 to 0.65 x 10(9)/l). Flow cytometry based on fluorescence-labelled monoclonal antibodies for monocyte enumeration seems an efficient tool to evaluate the monocyte counting performance of haematology analysers and an ideal successor to the manual differential as reference method for monocyte counting.