DIFFERENTIAL LEUCOCYTE COUNT 209 



Method of making a Differential Count. 



Having prepared a film by any of the methods previously 

 described, the next step is to make a differential count in the 

 following way : Focus the film under the microscope, using an 

 oil-immersion lens ; when you have had sufficient experience it 

 is quite easy to make the count with a |^-inch lens, which is 

 quicker, but not advisable for beginners. Then note down the 

 nature of each leucocyte as you come to it, moving the film 

 across the stage of the microscope from end to end, and then 

 moving it a little way upwards or downwards and returning in 

 the opposite direction, so as never to pass over the same part of 

 the film twice. The simplest way of noting down the leucocytes 

 is to assign single letters to each variety, P for polynuclear, E for 

 eosinophile, etc., and to put these down in blocks of five each 

 way, thus : 



P PP LE 



P P L L H 



LP P P P 



PL LP L 



PP PLP 



In this way you can tell at any time how many leucocytes you 

 have counted. It is much quicker to dictate the numbers to a 

 second person, who takes them down as described ; you do not 

 then have to look constantly from the microscope to the paper, 

 and vice versa, and the whole process takes a very few minutes, 

 unless the leucocytes are very scanty. 



For most clinical purposes 400 leucocytes will be enough to 

 count, though where very great accuracy is required 1,000 is 

 not too rnany. Having counted the required number, proceed to 

 count the numbers of P's, L's, etc., and reduce them to a 

 percentage. 



Whilst making the differential count keep a sharp look-out for 

 abnormal leucocytes, abnormal red corpuscles (see p. 213), 

 parasites, etc. The collections of blood platelets which form 

 such a prominent feature in some blood films should not be 

 confounded with anything else, as they are quite characteristic, 

 though a single blood platelet lying on the top of a red corpuscle 

 may look very like a young malaria parasite. Each platelet is a 

 very small mass which stains blue or purple with Jenner's stain, 

 and often appears hollow or irregular in shape, and they are often 

 grouped in masses of quite large size. 



