Sputum generally consists of inflammatory exudate from the lower respiratory tract mixed with saliva.
Mulder1 recognised the problem of interpreting the significance of growth from sputum and suggested rinsing it in saline before culture to remove the saliva. May2 showed that bacteria are often unevenly distributed in the sputum of patients suffering from chronic bronchitis and that single cultures may fail to reveal all the bacterial species present.
The introduction by Rawlins3 of a method for the homogenisation of sputum before culture overcame the variations present in any method that is based on the examination of small proportions of heterogeneous material. It enables the bacteria in the sputum to be distributed evenly throughout the specimen after digestion. Dixon and Muller4, in an attempt to distinguish between contaminants and bronchial pathogens, suggested a semi-quantitative analysis by diluting the digested sputum down to10-4.
Dithiothreitol, Cleland’s Reagent5, has been evaluated as a sputum liquefying agent6. It was found to be the most effective of a group of agents tested containing a sulphydryl group. A 0.1m solution of dithiothreitol was found to achieve a significantly greater decrease in sputum viscosity than 1.2M N- acetyl cysteine for use prior to sputum culture. The use of dithiothreitol instead of N-acetyl cysteine to digest sputum before decontamination has been shown7 to yield a higher number of acid-fast bacilli when smears are stained by the Ziehl-Neelsen method. After culture and incubation for three weeks, it was reported that, in general, the number and size of colonies isolated using dithiothreitol as a liquefying agent was greater than that using N-acetyl cysteine.
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