D4 Inflammatory and infectious markers

Topic

Inflammatory markers allow for the quantification of the systemic immune response's intensity and, alongside the direct detection of microorganisms, help identify infections, estimate their severity, and monitor their progression during treatment.

C-reactive protein (CRP) is synthesized by the liver, primarily in response to interleukin-6 released during inflammation. Its concentration begins to rise approximately 6 to 12 hours after the inflammatory process starts and peaks between 24 and 72 hours. Once the stimulus is removed, levels gradually return to normal over the following days. With effective treatment, CRP levels decrease in a relatively predictable manner; thus, monitoring its progression makes it possible to assess whether the inflammatory response is subsiding.

Procalcitonin is a more specific marker for distinguishing systemic bacterial infection from other causes of inflammation. Levels typically remain low during viral infections but rise rapidly in cases of bacteremia and sepsis, potentially reaching very high concentrations in severe cases. Once the infection is under control, the concentration decreases by approximately half every 24 hours, making it particularly useful for evaluating the response to antibiotics. A reduction of more than 80% from the peak level, or very low values, can support decisions to reduce or discontinue antibiotic therapy.

A suspicion of sepsis can also be established initially—without laboratory tests—using a rapid clinical tool based on three signs: a respiratory rate of 22 breaths per minute or higher, a systolic blood pressure of 100 mmHg or lower, and altered level of consciousness. The simultaneous presence of at least two of these criteria identifies patients at higher risk of deterioration or death, allowing for the prioritization of their assessment and treatment.

Unlike the aforementioned markers, blood cultures aim to directly detect bacteria or fungi present in the blood. The sample is placed in liquid media where microorganisms can multiply, while automated systems detect changes associated with their metabolism. The time required to obtain a positive result depends on the initial quantity of microorganisms and their growth rate. Some bacteria can be detected within a few hours, while others take longer, and fungi may require one to three days.

A single bottle does not detect all cases of bacteremia; therefore, collecting multiple bottles is recommended. The probability of detection increases when independent samples are combined, but one of the most important factors is the volume of blood cultured: the greater the appropriate volume obtained, the higher the likelihood of capturing microorganisms present in low concentrations. For this reason, two or three pairs of bottles are typically collected, with approximately 10 mL of blood in each.

A positive blood culture does not necessarily imply a true infection. Microorganisms that normally inhabit the skin can contaminate the sample during collection. To distinguish contamination from bacteremia, factors such as the type of microorganism, the number of positive bottles, the patient's clinical status, and the time it takes for each culture to turn positive are considered. The repeated presence of the same microorganism in multiple samples, combined with a compatible clinical picture, increases the likelihood of a true infection.

Together, these tools provide complementary information. PCR reflects the intensity and general progression of inflammation; procalcitonin offers greater specificity regarding bacterial infection and helps monitor the response to treatment; clinical screening allows for the rapid identification of patients at risk of sepsis; and blood cultures can directly demonstrate the presence of the causative microorganism.

Clinical practice combines these tools rather than relying on a single result. Monitoring the progression of PCR and procalcitonin levels helps determine whether the inflammatory or infectious process is responding to therapy; rapid clinical assessment aids in the early recognition of potential sepsis; and blood cultures allow for the identification of the causative agent and guide treatment decisions. Thus, the integration of markers, clinical signs, and microbiological detection makes it possible to identify an infection, estimate its severity, and verify whether the treatment is successfully controlling it.

ID:2278