When assessing a patient with suspected LRTI, secondary bacterial infections associated with COVID-19 pneumonia, or suspected sepsis, clinicians rely on a number of biomarkers and laboratory tests to make clinical decisions with confidence. Each marker offers its own unique advantages and disadvantages. The sensitivity and specificity for determining bacterial infection and severity is what sets procalcitonin (PCT) apart from the rest.
Traditional diagnostic tests used in the context of suspected infection—such as lactate, C-reactive protein, blood cultures, and white blood cell count—may not provide clear indication of bacterial infection. However, the results of these tests can be used alongside PCT testing to create a more defined picture of infection.
Let’s explore how PCT compares to, and complements, each of these biomarkers.
PCT and lactate are important biomarkers that offer information about two different pathophysiological processes. PCT levels indicate the likelihood of a bacterial infection and can help guide decision-making around initiating and discontinuing antibiotic treatment. In contrast, lactate levels reflect the status of tissue perfusion independently of the presence of an infection.
Moreover, while PCT is induced rapidly upon infectious insult, lactate does not rise until late in the course of sepsis.3 As tissue perfusion improves—e.g. due to fluid resuscitation—lactate levels should normalize.
For patients evaluated for a suspected bacterial infection or sepsis, the combination of lactate and PCT measurements—together with clinical data and vital signs—provides complementary information for risk stratification.4
C-reactive protein (CRP) is also a host response marker. Its secretion is triggered by cytokines in response to acute or chronic inflammation with high sensitivity. However, its specificity for bacterial infection is only moderate and the induction kinetics are slow, with increasing levels only after >12h after causal challenge, reaching peak after 36 to 50 hours .5-7
The long induction time of CRP, its lack of specificity for clinically relevant bacterial infection, and its suppression when corticosteroids are present have reduced its relevance when it comes to life-saving decision making.8 In contrast, PCT’s rapid induction time of 3-6 hours, its high sensitivity and specificity for systemic bacterial infection, and its decline in response to antibiotic therapy make it a valuable complement to clinical assessment and other biomarkers for sepsis risk assessment and patient management.9
Blood cultures are performed to identify bacteria, yeast, or other microorganisms that may be causing a patient’s clinical condition. Physicians then use the information to prescribe targeted treatments. Traditional blood culture takes several days (usually 2-3, but sometimes 10 days or longer, depending on the germ10-12), although more recent methods can provide a result in several hours.13
Given the slow turnaround time for cultures, physicians need to prescribe antibiotics empirically, as delaying antibiotic treatment in cases of true bacterial infection may increase the patient’s risk of progressing to a severe disease state. However, this decision comes at the cost of possibly increasing the chances of antibiotic overuse and resistance.
Another challenge with relying on blood cultures is interpreting the result correctly, as blood culture sensitivity is only moderate and may be further decreased in patients already on antibiotics. In addition, there is the potential for false-positive results due to sample contamination. Consequently, it can be difficult to determine whether the bacteria that grew in the culture are the cause of the clinical condition of the patient or not.10-12
With each PCT test taking only 20 minutes, results may be available in just 1-2 hours, especially if the hospital has good logistics in place. This information allows treating physicians to complement their clinical assessment in cases of suspected bacterial infection, including first decisions on the initiation of empiric, broad-spectrum antibiotic therapy. In addition to the PCT test, a blood culture helps identify the pathogen(s) and aid in narrowing the antibiotic spectrum.
During the further treatment course, PCT serial testing—combined with medical history and patient presentation—can be used for monitoring antibiotic treatment efficacy.
The white blood cell (WBC) count is probably the most commonly used biomarker for the consideration of an infection.4 An elevated WBC may be suggestive of an infection but acute trauma, burns, seizures, and some medications also can have a significant effect on the WBC. As a result, WBC offers low infection-specificity.4 This was supported by a study that found that more than 50% of patients diagnosed with sepsis had normal WBC.14
With the results from a PCT assay, clinicians can better assess the likelihood of bacterial infection and set patients up on a successful course of treatment with either appropriate antibiotics or alternative therapy.
In seeking the ideal marker or laboratory test for clinically relevant bacterial infection there are several important considerations. The perfect biomarker would:4