Sepsis is a life-threatening condition defined as organ dysfunction caused by the body’s overwhelming and dysregulated response to an infection. When the immune system overreacts, it can inadvertently damage the body’s own tissues and organs. Timely diagnosis is paramount, as every hour of delay in treatment significantly increases the risk of mortality. Because there is no single test for sepsis, a combination of time-sensitive blood tests and other diagnostic procedures must be used to measure systemic distress and identify the infectious source.
Immediate Blood Tests for Systemic Response
The initial blood tests quantify the degree of systemic shock and organ damage. The serum lactate level acts as a marker for tissue hypoperfusion. High lactate indicates that cells are not receiving enough oxygen, forcing them to switch to an anaerobic metabolic pathway that produces lactic acid. A level greater than 2 millimoles per liter (mmol/L) is a significant finding, and persistently high lactate suggests widespread cellular dysfunction and septic shock.
Another specialized marker is Procalcitonin (PCT), a protein precursor released into the bloodstream primarily in response to bacterial infections. PCT levels rise quickly, often within four hours of onset, peaking between 12 and 48 hours. Values below 0.05 nanograms per milliliter (ng/mL) are considered normal, while levels above 0.5 ng/mL suggest a higher likelihood of bacterial sepsis. This test helps clinicians differentiate a bacterial cause from non-infectious inflammation, guiding decisions about starting or stopping antibiotics.
A Complete Blood Count (CBC) provides a rapid snapshot of the body’s cellular response to infection. The white blood cell (WBC) count may be elevated as the body mounts an immune defense, though a low count can signal a severe, overwhelming infection. The platelet count is also closely monitored. A low platelet count, known as thrombocytopenia, can suggest coagulopathy or early organ failure, indicating severe systemic involvement.
Identifying the Site and Source of Infection
After initial tests confirm systemic distress, the next step involves pinpointing the cause and location of the infection. Blood cultures are the definitive test for identifying a bloodstream infection, requiring the collection of at least two separate sets of blood samples before administering antibiotics. Even in confirmed sepsis cases, blood cultures yield a positive result in only about 30 to 40 percent of patients.
To avoid treatment delays, broad-spectrum antibiotics are started empirically while the cultures incubate, a process that can take up to 72 hours for final identification and susceptibility testing. Cultures of other body fluids are also collected based on the suspected infection site to identify pathogens originating there. These may include:
- Urine cultures if a urinary tract infection is suspected.
- Samples of cerebrospinal fluid.
- Sputum samples.
- Wound swabs.
Imaging studies are utilized when the source of the infection is not clear from the physical examination. A chest X-ray can quickly reveal signs of pneumonia, a common cause of sepsis. More advanced imaging, such as a Computed Tomography (CT) scan or an Ultrasound, may be required to visualize internal abscesses, fluid collections, or other anatomical sources of infection. Identifying the location of the infection is necessary for guiding treatment, which often includes a procedure to drain the infectious source in addition to antibiotic therapy.
Interpreting Results for Treatment Decisions
The diagnosis of sepsis relies on synthesizing multiple data points, including laboratory findings, vital signs, and clinical assessment. Clinicians use screening tools like the Quick Sequential Organ Failure Assessment (qSOFA) score to rapidly identify high-risk patients outside of the intensive care unit. This score assigns points for a high respiratory rate, altered mental status, and low blood pressure, indicating potential organ dysfunction that warrants immediate laboratory workup.
Once sepsis is suspected, the accumulated results guide the time-sensitive initiation of the sepsis care bundle. This protocol mandates the prompt administration of intravenous fluids to address hypoperfusion and broad-spectrum antibiotics to target the likely pathogen, ideally within the first hour. The choice of antibiotic is often adjusted, or “de-escalated,” once the specific bacteria and its sensitivities are identified from the slower-growing culture results.
Continuous monitoring of the patient’s response relies heavily on repeat blood testing, particularly the measurement of lactate clearance. This calculation tracks the percentage decrease in lactate levels over a defined period, such as six hours, following the start of resuscitation. A significant drop in lactate indicates that the body is responding positively to treatment, meaning tissue perfusion is improving and cellular oxygenation is normalizing. Conversely, persistently elevated lactate is a poor prognostic sign, signaling that the patient’s condition remains unstable and requires further interventions.