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Sputum consists of a mixture of cellular and non-cellular elements cleared by mucociliar apparatus. It is easy to obtain and cause little or no patient discomfort, but its use is declining because of the advent of bronchoscopy and fine needle aspiration.
The sensitivity of sputum cytology for the diagnosis of malignancy increases with the number of specimens examined (from 40% with a single specimen to 90% with five specimens). Multiple samples are collected over several days. Early morning, deep cough specimens are required. Expectoration can be induced by having the patient inhale nebulized saline.
The specificity is very high, but negative results do not guarantee the absence of a malignancy. The sensitivity of sputum cytology also depends on the location of the tumor: 75% of central lung cancers and only 45% of peripheral cancers are diagnosed. The more peripheral the tumor, the fewer the diagnostic cells that will be found in the sputum and the more difficult the cells are to obtain bronchoscopically. However, bronchial cytology is generally superior to sputum cytology in the diagnosis of peripheral lesions.
The postbronchoscopy sputum specimen may have the highest diagnostic rate of any respiratory cytology specimen and it may be the only positive specimen when preceding sputum, bronchial washing, or brushing samples are all negative.
Specimen adequacy is determined by the presence of several alveolar macrophages. Specimens only consisting of squamous cells from the upper respiratory tract are unsatisfactory.
In the pick and smear technique fresh sputum is examined for tissue fragments or blood. Smears are prepared from areas containing these elements and fixed in 95% ethanol. When prompt preparation is not possible, the patient should prefix the specimen by expectorating in a 70% ethanol solution. A modification of this method is the Saccomanno method: sputum is collected in 50% ethanol and 2% carbowax (polyethylene glycole) and the specimen is homogenized in a blender and concentrated by centrifugation. This process must be performed in a biologic safety hood in order to avoid infection by aerosolization. Smears are obtained from the concentrated cellular material.
Sputum can also be processed using thinlayer methods or embedded in paraffin for cell block sections.
