Bacterial density in sputum : is it reliable ?

Question

Hello,
What does the density of bacteria found in the sputum mean (result given by the microbial laboratory versus the lung reality)? And then, when extrapolating, is a variation of the bacterial density (increase or decrease within 6 months for instance) representative of the amount of bacteria into the bronchi? Is this density a scientific criterion for deciding a intravenous antibiotics course, inhalative therapy or other strategies?
Thanks for your response.

Answer

Hello and thanks for your question for which responses are not easy because of three problems to discuss: the sputum sample quality, the microbial analysis per se and its clinical interpretation.
To obtain secretions that perfectly reflect the bronchial flora is possible only via a bronchial fibroscopy, that is of course a procedure that we can’t realize at each scheduled medical visit. That’s why the large majority of the samples are obtained from sputum in spluttering patients, but this procedure only reflects the bacterial flora from the beginning of the lower airways (trachea and perhaps the main bronchi). In young children who don’t splutter a nasopharyngeal aspiration is possible, but obtained secretions of course less reflect bronchial flora.
Obtaining a bacterial density from the microbial laboratory supposes to perform a quantitative analysis of the bacteria which is only possible for fluid or semi-fluid samples. It’s easy to understand that this technique is easier in the oldest spluttering children and adults than in the youngest ones. The lower threshold for bacterial detection is usually 10 power 2 CFU (colonies forming unit) and requires 10 µl of a homogenized sample in an adequate growing environment. The results are robust and reproducible, with one log 10 difference.
The presence of bacteria in sputum has to be interpreted cautiously and depends from the bacteria itself and from the clinical ambiance. For instance when Pseudomonas aeruginosa is found for the first time, a treatment is usually proposed independently of the bacterial density and of the patient clinical symptoms because the aim of the treatment is to eradicate this germ. If this bacteria is irregularly found in the patient's sputum samples, only at some occasions, the treatments will be proposed in case of patent clinical signs (change in cough, increase in sputum volume, loss of appetite or weight, etc.) or in case of elevated bacterial density and possible clinical signs. If the bacteria are regularly found in the sputum, sometimes in each sputum sample, antibiotics will be often proposed systematically every 3-4 months by intravenous courses, and an inhalative therapy will be given between these courses to contain at the best the bacteria.
I hope that I have answered to your question. Best regards.
Pr Jean-Christophe Dubus

10.10.2011