Sepsis is the body’s response to an infection that can lead to tissue damage, organ failure, and death. To identify the microorganism causing the infection, clinicians use a blood culture. This test involves placing a blood sample into a medium that promotes the growth of germs like bacteria or fungi, allowing for their identification.
Sometimes, a patient shows all the signs of sepsis, yet their blood culture is negative. This condition is known as culture-negative sepsis. It is challenging for healthcare providers because the primary tool for identifying the infectious agent does not provide a definitive answer, leaving them to treat a severe infection without knowing its specific cause.
Understanding a Negative Culture Result
A common reason for a negative blood culture is the administration of antibiotics before the blood sample is drawn. A single dose can suppress bacterial growth in the culture, preventing detection. For this reason, obtaining blood cultures before starting antibiotic therapy is standard procedure in managing suspected sepsis.
The infecting pathogen can also lead to a negative result. Standard blood cultures are designed to grow common bacteria, not viruses or fungi. Specialized tests, which are not always performed initially, are required to detect these types of infections.
Some bacteria are “fastidious,” meaning they have complex nutritional needs and are difficult to grow in a lab. These organisms may not multiply enough during the standard incubation period, leading to a false-negative result. The infection might also be localized to a specific organ or tissue, like a liver abscess, meaning the number of bacteria in the bloodstream may be too low for a blood culture to detect.
Diagnosis Beyond the Culture
Without a positive blood culture, diagnosing sepsis relies on clinical evaluation and other lab tests. Physicians assess the patient for characteristic signs of sepsis, which are part of standardized criteria used to identify potentially septic patients. These indicators include:
- An elevated heart rate (tachycardia)
- Rapid breathing (tachypnea)
- An abnormal body temperature (fever above 38.3°C or low temperature below 36°C)
- An altered mental state, such as confusion or disorientation
Biomarkers are also used in the diagnostic process. Two biomarkers for sepsis are procalcitonin (PCT) and C-reactive protein (CRP). The levels of these proteins rise in response to a bacterial infection, and high levels can support a sepsis diagnosis when a blood culture is negative. Another biomarker is lactate, which is measured to assess tissue perfusion; elevated lactate indicates that organs may not be receiving enough oxygen, a sign of severe sepsis.
Imaging studies are used to locate the source of an infection when it is not apparent. A chest X-ray can reveal pneumonia, while CT scans of the abdomen and pelvis can identify abscesses or organ inflammation. Ultrasound can also be used to examine specific organs, like the gallbladder or kidneys, for signs of infection.
Treatment Strategies
Treatment for culture-negative sepsis is initiated based on clinical suspicion, without waiting for culture results. The immediate priority is to start broad-spectrum antibiotics. These are antibiotics effective against a wide range of the most common bacteria that cause sepsis, ensuring treatment begins as quickly as possible.
Supportive care is administered alongside antibiotic therapy to manage the body’s response to the infection. Intravenous (IV) fluids are given to restore hydration and maintain blood pressure. Oxygen therapy is also provided to ensure the blood is adequately oxygenated and protect organs from damage.
In cases of septic shock where blood pressure remains low despite IV fluids, medications called vasopressors are used. These drugs constrict blood vessels to raise and stabilize blood pressure, ensuring blood flow to the organs. If a specific source like an abscess is identified, surgical drainage or removal of infected tissue may be necessary.
Prognosis and Patient Outlook
The prognosis for patients with culture-negative sepsis is varied when compared to culture-positive sepsis. Some research indicates that mortality rates are similar between the two groups, while other studies find differences based on the patient population and illness severity. For instance, one study found that among patients with septic shock, those with negative cultures had higher mortality rates.
A patient’s outlook depends on several factors, including their overall health before sepsis, how quickly treatment was initiated, and the underlying cause of the infection. A patient with a viral infection may have a different course than one with a bacterial infection that was difficult to culture.
Sepsis is a medical emergency, regardless of the culture result. Culture-negative sepsis is as serious as culture-positive sepsis and carries a high risk of severe complications and death. Prompt treatment provides the best chance for a successful recovery, which highlights the importance of seeking immediate medical attention if sepsis is suspected.