Briefly discuss the diagnostic value of expectorated sputum in establishing the aetiology of infections of the lower respiratory tract. Briefly comment on the alternative specimens which may prove useful in establishing the aetiology of such infections.

Outline:
•	Bronchitis: culture for bacteria & serology. 
•	Pneumonia: culture of bacteria & serology.
•	Alternative specimens: blood & spinal fluid.

Suggested Answer:

	Infections of the lower respiratory tract include bronchitis, bronchiolitis and pneumonia. These syndromes, especially pneumonia, can be severe or fatal. Although viruses, mycoplasma, rickettsiae and fungi can all cause lower respiratory tract infections, bacteria are the dominant pathogens.
 
Viruses cause most cases of bronchitis and bronchiolitis. Mycoplasma pneumoniae is a common cause of acute bronchitis while Streptococcus pneumoniae and Haemophilus influenzae have been implicated in cases of chronic bronchitis. Bacteriologic examination and culture of purulent respiratory secretions should always be performed for cases of acute bronchitis not associated with a common cold. Patients with chronic bronchitis should have their sputum cultured for bacteria initially and during exacerbations. Aspirations of nasopharyngeal secretions or swabs are sufficient to obtain specimens for viral culture in infants with bronchiolitis. Serologic tests demonstrating a rise in antibody titer to specific viruses can also be performed. Rapid diagnostic tests for antibody or viral antigens may be performed on nasopharyngeal secretions by using fluorescent-antibody staining, ELISA or DNA probe procedures. 

Sputum cultures are performed primarily when pneumonia, tuberculosis, or lung abscess is suspected. The most frequent cause of community-acquired pneumonia is Streptococcus pneumonia¸ whereas gram-negative rods, such as Klebsiella pneumoniae, are common causes of hospital-acquired pneumonias. Sputum should be examined for a predominant organism in any patient suspected to have a bacterial pneumonia; blood and pleural fluid (if present) should be cultured. A sputum specimen with fewer than 10 while cells per high-power field under a microscope is considered to be contaminated with oral secretions and is unsatisfactory for diagnosis. 

Culture of the sputum on blood agar frequently reveals characteristic colonies, and identification is made by various serologic or biochemical tests. Acid-fast stains and cultures are used to identify Mycobacterium and Nocardia spp. Most fungal pneumonias are diagnosed on the basis of culture of sputum or lung tissue. Viral infection may be diagnosed by demonstration of antigen in secretions or cultures or by an antibody response. Serologic studies can be used to identify M pneumoniae, C. burnetii, Chlamydia species, Legionella, Francisella,and Yersinia. A rise in serum cold agglutinins may be associated with M pneumoniae infection, but the test is positive in only about 60% of patients with this pathogen. Enzyme-linked immunoassay, DNA probe and polymerase chain reaction methods are available for many agents causing respiratory infections. Some organisms that may colonize the respiratory tract are considered to be pathogens only when they are shown to be invading the parenchyma. Diagnosis of pneumonia due to cytomegalovirus, herpes simplex virus, Aspergillus spp. or Candida spp require specimens obtained by transbronchial or open-lung biopsy. Pneumocystis carinii can be found by silver stain of expectorated sputum. However, if the sputum is negative, deeper specimens from the lower respiratory tract obtained by bronchoscopy or by lung biopsy are needed for confirmatory diagnosis. 

Alternative specimens are collected when there are other pathologic conditions co-existing with the low respiratory tract infections such as meningitis, endocarditis and osteomyelitis, or if it is suspected that the causative agent is not of the common pathogens. Additional blood cultures are performed most often when sepsis, endocarditis, osteomyelitis and meningitis are suspected. The organisms most frequently isolated from blood cultures are Staphylococcus aureus, Klebsiella pneumoniae and Pseudomonas aeruginosa. spinal fluid cultures are performed primarily when meningitis is suspected. Streptococcus pneumoniae is often the aetiological agent involved.