Summary The Coulter Counter Model S Plus is a 12 parameter haematological analyser designed for service use in haematology laboratories. Eight parameters are standard in current routine haematological practice; the seven parameters generated by the Model S and a platelet count. The method of platelet counting is unique. The remaining parameters are new and comprise the platelet‐crit, the mean platelet volume and size distribution measurements for both platelets and red cells. A description of the instrument is given including differences from the Model S. The new parameters are discussed in detail. Instrument precision is assessed in terms of linearity, reproducibility, drift, carry‐over and protein buildup. The results of all are impressive. Instrument accuracy is assessed in detail; white cell count, red cell count, haemoglobin concentration and mean corpuscular volume being compared with those values measured by the Model S; the Model S Plus haematocrit is compared with the microhaematocrit and platelet counts with those from the Thrombocounter C/Thrombofuge system. All correlations are very satisfactory. Normal values are defined for the new parameters. Instrument design and function are assessed and reagent consumption quoted. Cell control reagents have been evaluated. A realistic hourly throughput for the Model S Plus is 70–80 samples.

suMMARY Coulter Electronics Ltd have produced a semi-automated platelet-counting system. Platelet-rich plasma may be obtained either by tube sedimentation or by means of the Thrombofuge, the latter being an instrument designed to produce accelerated sedimentation. The Coulter Thrombocounter C is a special purpose version of the Coulter Counter, rationalised and simplified to count platelets, and comprises integral sampling and electronic assemblies. The orifice selected in this system has a 70 micron diameter. The orifice diameter is determined by three factors, namely, the range of platelet size, electronic and biological noise at the lower end of the system, and, at the other extreme, larger non-platelet particles. Thus the useful range of orifice is 4-40 % of the total diameter. The threshold controls on the Thrombocounter C are pre-set for 3-30 cubic microns, figures which satisfy the physical require-

A comparison has been made between Coulter Counter ZBI/Channelyser C1000 derived platelet distribution width (PDW), mean platelet volume (MPV) and platelet crit (PCT) with similar measurements generated by the Coulter counter Model S Plus. Correlations of PDW and MPV from the two systems are poor; that for PCT is acceptable. The differing results can almost certainly be attributed to physical differences in the measurement systems.

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