The Cobas-Helios (Roche Diagnostic Systems, Inc., Branchburg, NJ) is a new, fully automated hematology analyzer that performs a complete blood count and differential leukocyte count (DLC), classifying leukocytes by flow-cytochemical technology. The DLC component of the Cobas-Helios was evaluated according to the National Committee for Clinical Laboratory Standards H20-A protocol. Instrument performance was acceptable with respect to all parameters investigated, including imprecision, inaccuracy and clinical sensitivity for the identification of quantitative and qualitative leukocyte abnormalities. In a minority of samples with neutrophil left shift, neutrophils tended to overlap the monocyte domain, resulting in overestimation of monocytes and underestimation of neutrophils. This problem did not affect clinical sensitivity and was generally associated with a positive instrumental left-shift flag. Flags for the identification of specific qualitative abnormalities of the leukocyte population (atypical lymphoid cells, nucleated red cells, blast cells, immature granulocytes and neutrophil left shift) performed well. In addition to a conventional five-part DLC, the Cobas-Helios also identifies and quantitates atypical lymphoid cells and "large immature cells," the latter corresponding to bands and immature granulocytes. Counts of atypical lymphoid cells and large immature cells correlated well with the equivalent cell classes as enumerated by the reference method of the National Committee for Clinical Laboratory Standards. The Cobas-Helios offers the most reliable quantitative index of neutrophil left shift currently available in a commercial automated DLC analyzer.

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