43 
Leucocytic Variation .—Often in cases where pig¬ 
mented leucocytes are difficult to find there is a very 
obvious increase in the percentage of the large mono¬ 
nuclear leucocytes. This change, which is usually very 
pronounced in the apyretic periods of an attack of 
malaria, is, however, most frequently absent during 
pyretic periods. If, during a period of low tempera¬ 
ture, this change is not found, there is a strong pre¬ 
sumption that the case is not malarial. If the blood 
be taken at the height of the fever, a negative result 
does not exclude malaria, and a further examination 
should be undertaken, if possible, during an apyretic 
period. In some cases the change can be detected 
even during the pyretic periods, but in these it is 
always more marked in the apyretic. In some cases, 
during the course of the fever, no such change occurs, 
but appears immediately the temperature subsides, 
and diminishes as convalescence proceeds. Perhaps 
the cases where this test is of the greatest value are 
those where the patient has already been treated with 
quinine, and one can scarcely hope, even if the disease 
be malaria, to find parasites in the blood. 
From the results obtained by blood counts of a 
considerable number of Europeans living in the tropics, 
we found that an increase beyond fifteen per cent, of 
the large mononuclear forms is proof of an actual or 
recent malarial infection, whereas with a value of 
twenty per cent, it is almost always possible, by long 
search, to find an occasional parasite or pigmented 
leucocyte. A value of over twenty per cent, probably 
implies actual infection at the time of observation. 
