An Integrated Approach to Diagnosis

Although the differential diagnosis of lung disease in a person with HIV infection is quite broad, the probabilities of the various diagnoses can be reduced in a given patient by knowing the patient's symptoms, HIV transmission category, and CD4+ lymphocyte count and further refined by the findings on the chest film.

The first step in the diagnostic evaluation is to define the patient's symptoms, especially whether or not the patient has cough and/or shortness of breath. If cough is present, it is important to ascertain if it is productive of purulent sputum. It is very uncommon for patients with PCP to have purulent sputum. Moreover, depending on the stain used, the presence of purulent debris in respiratory tract specimens makes it difficult to detect P. carinii in smears of sputum or bronchoalveolar lavage (BAL) fluid. As noted previously, acute bronchitis and bacterial pneumonia are relatively more frequent among persons with higher CD4+ lymphocyte cell counts. Additionally, IDUs have higher rates of bacterial pneumonia and tuberculosis than persons in other HIV transmission categories. Also as noted, the differential diagnosis varies somewhat with the geographic area, with histoplasmosis and coccidioidomycosis being common in their respective endemic areas.

As in all patients with significant respiratory symptoms, radiographic examination of the chest is usually the first test performed. If the chest film shows no abnormalities in a patient with purulent sputum, the likely diagnosis is bronchitis. Patients with a diagnosis of bronchitis should be followed to be certain that the symptoms resolve and, if there is no resolution or worsening, further evaluation should be undertaken. It must be kept in mind that both tuberculosis and PCP may present with normal chest films.

Patients who have normal chest films and a nonproductive cough may also have bronchitis, but if the CD4+ lymphocyte count is < 300 cells per microliter, P. carinii should be considered. In this circumstance, pulmonary function testing with measurement of the diffusing capacity for carbon monoxide (DL CO) should be performed. If the DL CO is < 75% of the predicted normal value, further evaluation directed toward detecting P. carinii should be undertaken. An alternative approach is to replace measurement of the DL CO with thin-section CT scanning, using a limited number of images to reduce cost. This approach has the potential advantage of being able to distinguish among PCP, emphysema and vascular obliteration caused by foreign particle embolization from intravenous drug use.

If the chest film is abnormal, the next step depends on the type of abnormality. Focal infiltration, especially consolidation, in a patient with purulent sputum is most consistent with a diagnosis of bacterial pneumonia or tuberculosis. Sputum should be obtained for Gram stain, acid-fast stain, and cultures for pyogenic organisms and mycobacteria. If there is a poor response or worsening, further evaluation, especially for P. carinii, should be performed.