A sputum culture is a laboratory test that examines the thick mucus, or sputum, produced deep within the lungs and respiratory airways. This material is distinct from watery saliva or nasal discharge, as it often contains the infectious agents causing a lower respiratory tract infection. The culture allows microorganisms to grow in a controlled environment, enabling technicians to identify the specific pathogen responsible for an illness. Since the test relies on collecting material from the lungs, not the mouth, the quality of the sample directly affects the accuracy of the diagnostic results. Contamination with too much saliva can lead to inaccurate findings and delay appropriate patient treatment.
Understanding the Purpose of the Sputum Culture
Healthcare providers order a sputum culture when they suspect an infection in the lungs, such as pneumonia, bronchitis, or tuberculosis. The culture serves as a tool to precisely identify the infectious agent, which can be bacteria, fungi, or mycobacteria, including Mycobacterium tuberculosis. By pinpointing the exact type of germ, the test moves beyond a general diagnosis of a respiratory infection to a specific microbial cause. This identification is fundamental because it guides the selection of the most effective therapy. For bacterial infections, the laboratory often performs susceptibility testing alongside the culture, which determines which antibiotics will successfully kill the identified organism.
Essential Preparations Before Collection
The quality of the specimen collected is highly dependent on the steps taken immediately beforehand to minimize contamination from the mouth’s natural flora. The ideal time for collection is first thing in the morning, right after waking up. Secretions tend to accumulate in the lungs overnight, making it easier to produce a concentrated sample with higher diagnostic value. The patient must first rinse their mouth thoroughly with plain water to remove loose food particles and surface bacteria. It is important to avoid using mouthwash or toothpaste, as the antimicrobial agents in these products can kill the pathogens being tested for, leading to a false-negative result.
Step-by-Step Guide to Collecting a Quality Sample
The physical act of collecting the sample requires a forceful technique to ensure the material comes from the lower airways and not just the throat. The sterile, wide-mouthed collection container should be opened only when the patient is ready to cough, and the inside of the container and lid must not be touched to prevent contamination. The patient should take several slow, deep breaths, filling the lungs completely, which helps to loosen the secretions.
After the deep breaths, the patient must perform a forceful, deep cough that originates from the chest, not a clearing of the throat. The cough should be explosive and strong enough to bring the material up from the lungs and into the mouth. The expectorated material, which is the true sputum, should be thick and mucoid, sometimes discolored yellow, green, or even blood-tinged, clearly distinguishing it from thin, clear saliva.
The patient should immediately spit the sputum directly into the sterile container, taking care to avoid getting material on the outside of the container. The process of deep breathing and coughing may need to be repeated multiple times to obtain a sufficient volume. For most cultures, the laboratory requires a minimum of 5 to 10 milliliters of sputum, which is roughly equivalent to one or two teaspoons. Collecting an insufficient sample or one that is mostly saliva will result in the lab rejecting the specimen for testing.
Proper Handling and Transport of the Specimen
Once the required amount of sputum is collected, the container must be sealed immediately and tightly closed to prevent any leakage or contamination during transport. The next step involves accurately labeling the specimen container with the patient’s full name, date of birth, and the specific date and time of collection. The specimen must then be delivered to the laboratory as quickly as possible, ideally within one to two hours of collection, to maintain the viability of the microorganisms. If there is an unavoidable delay in transport, the specimen should be temporarily stored in a refrigerator at a temperature between 2 and 8 degrees Celsius. Refrigeration helps to slow the growth of normal mouth bacteria, preserving the integrity of the pathogens of interest for up to 24 hours.