192 CLINICAL BACTERIOLOGY AND HEMATOLOGY 



cases of typhoid fever referred to above the haemoglobin is usually 

 slightly lowered, showing that there is anaemia, even where the 

 number of corpuscles is abnormally high. 



In the diagnosis of the nature of an ansemia the enumeration of 

 the red corpuscles is necessary, and the result is to be considered 

 in connection with the percentage of haemoglobin. The colour- 

 index, or index of corpuscular richness, is the figure which indicates the 

 richness of each corpuscle in haemoglobin, the normal figure being 

 unity. It is obtained by dividing the amount of haemoglobin by 

 the number of corpuscles, each being expressed as a percentage 

 of the normal amount. In health there are 5,000,000 red 

 corpuscles = 100 per cent, of normal and 100 per cent, of haemo- 

 globin. The index is therefore : 



100 

 100 ~ 



If we found a case in which the red corpuscles were 4,000,000 

 ( = 80 per cent, of normal) and the haemoglobin 40 per cent., the 

 colour-index would be : 



40 



In another case we might find : — Red corpuscles 1,000,000 



( = 20 per cent, of normal), and hsemoglobin 25 per cent. The 



colour-index is : 



25 



- =1-25. 



20 -^ 



These figures would be suggestive of chlorosis and pernicious 

 anaemia respectively. 



The percentage of red corpuscles is obtained from the absolute 

 count by multiplying the millions figure and the hundred thousand 

 figure by two (since the normal is 5,000,000, the said figures of 

 which become 100 when multiplied by two). Thus, 2,500,000 

 = 50 per cent. ; 900,000 =18 per cent., etc. The same rule may be 

 used for women, for the slightly lower normal total of reds is 

 accompanied by a smaller amount of haemoglobin. 



The following are the general rules (to which exceptions are 

 rare) for the interpretation of the colour-index : 



1. An index decidedly above unity indicates pernicious anaemia, 

 and usually in this disease the greater the anaemia the higher the 

 index. 



2. In chlorosis the index is greatly diminished : o"2 has been 

 recorded, and the average is about 0-5. The exact figure throws 



