Chapter 13: Diagnostic tests
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Evidence‐based approaches
Rationale
The main aim of sputum or secretion collection is to provide reliable information on the causative agent of bacterial, viral or fungal infection within the respiratory tract and its susceptibility to antibiotics, in order to guide treatment (Chernecky and Berger [32], Pagana and Pagana [177], PHE [191]).
Indications
A respiratory tract secretion specimen is indicated:
The presence of sputum, especially when discoloured, is commonly interpreted to represent the presence of bacterial infection and seen as an indication for antibiotic therapy. However, purulence primarily occurs when inflammatory cells or sloughed mucosal epithelial cells are present, and can result from either viral or bacterial infection (Chernecky and Berger [32], PHE [191]). One strategy for limiting or targeting antimicrobial prescribing is to send a respiratory tract specimen for microbiological analysis to either demonstrate that a substantial infection is not present or identify an organism for which antimicrobial treatment is deemed necessary.
Methods of non‐invasive and semi‐invasive sampling
There are various methods of non‐invasive and invasive sampling methods, from self‐expectoration to a bronchoalveolar lavage. A sufficient quality of sputum will yield a representative sample; early morning sputum samples are preferred as they contain pooled overnight secretions in which pathogenic bacteria are more likely to be concentrated (PHE [191], Philomina [194]).
Self‐expectoration
For patients who are self‐ventilating, co‐operative and able to cough, expectorate and follow commands, a sputum sample is a suitable collection method. Sputum produced as a result of infection is usually purulent and a good sample can yield a high bacterial load. Ideally at least 1 ml of sputum should be collected (PHE [191]). In cases of suspected Mycobacterium tuberculosis, three sputum specimens collected on at least three consecutive days are required (as the release of the organism is intermittent) before the pathogenic organisms can be isolated (Damani [40], PHE [191]). See Procedure guideline 13.27: Sputum sampling.
Nasopharyngeal samples
Nasopharyngeal suctioning is a viable alternative for patients who are obtunded or whose cough is weak (Chernecky and Berger [32], PHE [191]). Sampling techniques to obtain specimens from the nasopharynx are semi‐invasive but can be used on patients who are self‐ventilating. They are indicated in suspected viral infections such as respiratory syncytial virus (RSV), influenza and parainfluenza. The main aim is to collect epithelial cells from the posterior nasopharynx and a sample can be obtained using nasal washing or vacuum‐assisted aspiration (see Procedure guideline 13.28: Nasopharyngeal wash: syringe method). This method can yield a more reliable sample because the normal flora present in the oropharynx are bypassed (Philomina [194]).
Methods of invasive sampling
Intubated patients who are unable to clear secretions independently or who require more accurate specimen collection may need a more invasive technique to obtain a sample. Invasive techniques obtain secretions directly from the lower airway and are designed to avoid contamination by upper airway colonization, which may lead to misinterpretation of cultures (PHE [191]).
Vacuum‐assisted aspirate via endotracheal tube
Endotracheal suctioning is frequently used as a diagnostic method and to obtain specimens in intubated patients with suspected pulmonary infection. This technique bypasses the upper respiratory airways and provides an accurate microbiological result. Suctioning aids the clearance of secretions by the application of negative pressure through a sterile flexible suction catheter or a closed suction system. A sterile sputum trap is attached to the suction catheter at one end, while the other end is attached to the suction tubing to collect the sample (Figure 13.25).
Figure 13.25 Types of vacuum‐assisted sputum trap.
Bronchoalveolar lavage
Bronchoalveolar lavage (BAL) is a reliable and accurate technique that provides a good diagnostic yield in cases of pulmonary infection, particularly invasive fungal infections and malignancies (PHE [191]). BAL specimens are particularly useful in immunocompromised patients with diffuse pneumonia (Brooks et al. [24]). This method is used to clear secretions and to obtain washings for the retrieval of cellular and non‐cellular components of the epithelial surface of the lower respiratory tract (PHE [191]). Endotracheal aspiration can eliminate some of the contamination problems of expectoration in the sampling of sputum.
Flexible fibreoptic bronchoscopy is a relatively safe and well‐tolerated means by which to obtain BAL fluid (Chernecky and Berger [32], PHE [191]). A flexible bronchoscope is inserted through the endotracheal tube and advanced distally into a subsegmental bronchus. The area selected for sampling is based upon a correspondent area of infiltration on a chest X‐ray or by visualization of a subsegment containing purulent secretions. A volume of sterile 0.9% sodium chloride (normally 20–30 mL) is then instilled through a port on the bronchoscope and suctioned back through another port into a sterile sputum trap. The procedure is repeated three to five times and the samples are then dispatched to the laboratory for microscopy and microbiological analysis (Chernecky and Berger [32], Keogh [114], Pagana and Pagana [177], PHE [191]).
