**3. Laboratory and other diagnostic tests**

Sputum examination is one of the best ways of differentiating bacterial pneumonia from pulmonary infarction. In bacterial pneumonia the sputum classically is purulent, occasionally foul smelling and may contain bright red fleck of blood. Gram's stain typically shows many bacteria and polymorphonuclear leukocytes. In pulmonary infarction, sputum, when present, usually is frankly blood with few bacteria or inflammatory cells. If the infarct becomes infected the sputum may be indistinguishable from that in bacterial pneumonia. Blood cultures often reveal the causative microorganism in the patients with bacterial pneumonia but show no growth in cases of bland pulmonary infarction (8).

Cavitation after bland pulmonary infarcts may result from either aseptic necrosis of the infracted lung or from secondary bacterial infection with subsequent abscess formation. It is infective almost as often as it is aseptic (9).

Two types of infected pulmonary infarct have been proposed based on the mode by which infection sets in (10). One is called "primary" because the infection is from a septic embolus. The other is called "secondary" because the infection is bronchigenic origin. Some authors suggest that the development of fever and/or purulent sputum following a pulmonary infarct is highly suspicious for secondary infection. The spectrum of causative agents for infected pulmonary infarct is similar to that of nosocomial pneumonia (3).

Total leukocyte count has limited discriminatory value. It usually is normal or slightly elevated in pulmonary infarction, but there is reports of leukocytes count higher than 40 000 per mm3 in patients with massive, bland necrosis of pulmonary tissue. Elevated serum lactic dehydrogenase (LDH) activity, normal aspartate aminotranspherase activity (AST) and increased serum bilirubin concentration forms a triad once considered a sensitive indicators of pulmonary embolism and infarction. However, subsequent studies have shown that these tests fail to differentiate pulmonary infarction from pneumonia and host of other disorders (8).

Electrocardiographic abnormalities that may appear are right ventricular conduction disturbances; right axis deviation; inverted T-waves with S-T segment deviation in the right precordial leads; peaked T-waves in leads II, III and AVF; various types of supraventricular and raraly ventricular arrhythmias and S1, Q3 or S1,S2, S3 patterns. Heart ultrasound may reveals right ventricular dilatation, septal deviation of left ventricle, tricuspidal regurgitation or right ventricle hypokinesis with wall thinning (8, 11).

Pulmonary function test and arterial blood gas studies provide data that are too variable and nonspecific to differentiate bacterial pneumonia from pulmonary infarction (8).
