From the Guidelines

An elevated procalcitonin level indicates a high likelihood of bacterial infection or sepsis.

Procalcitonin (PCT) is a precursor hormone of calcitonin produced by the parafollicular cells of the thyroid gland and neuroendocrine cells of the lung and the intestine that are thought to discriminate the systemic response due to bacterial causes from viral and noninfectious etiologies 1.
PCT begins to rise four hours after exposure to bacteria, reaching a maximum level after six to eight hours, and serum levels of PCT are associated with the severity of the infection, and decrease rapidly after antibiotic treatment 1.
A recent systematic review and meta-analysis evaluated the diagnostic accuracy of PCT and CRP in the diagnosis of sepsis in adults, and the overall area under the summary receiver operator characteristic (SROC) curve was 0.85 for PCT, with a sensitivity and specificity of 0.80 and 0.77, respectively 1.
Several RCTs have demonstrated that PCT-based algorithms safely reduce antibiotic use in stable, low-risk patients with respiratory infections, and PCT levels of less than 0.5 µg/L or levels that decrease by greater than or equal to 80% from peak levels may guide antibiotic discontinuation once patients stabilize 1.
The use of PCT-guided antibiotic treatment in ICU patients with infection and sepsis patients resulted in improved survival and lower antibiotic treatment duration, according to a meta-analysis of 11 RCTs involving 4,482 patients 1.

It is recommended to initiate or continue broad-spectrum antibiotics, such as Piperacillin-Tazobactam 4.5g IV every 8 hours or Meropenem 1g IV every 8 hours, and provide supportive care, including fluid resuscitation and monitoring of vital signs.
Consider consulting an infectious disease specialist for further guidance.
PCT and CRP provide only supportive and complementary information to clinical assessment, and decisions on initiating, altering, or discontinuing antimicrobial therapy should not be made solely based on changes in PCT or CRP levels 1.

From the Research

Elevated procalcitonin levels can indicate several conditions, including:

Sepsis, severe sepsis, and septic shock 2
Bacterial infection, with higher levels indicating Gram-negative sepsis 3, 4
Non-bacterial inflammatory conditions, such as non-specific inflammation and neoplastic diseases 5
Higher risk of mortality within 28 days in cases of bacteremia 4

Procalcitonin levels can vary depending on the underlying condition:

Higher levels in Gram-negative sepsis compared to Gram-positive and fungal sepsis 3
Higher levels in bacteremia cases with Gram-negative rods, such as Escherichia coli, compared to Gram-positive rods or Gram-positive cocci 4
Positive correlation with serum levels of C-reactive protein, soluble interleukin-2 receptor, and ferritin 5

Procalcitonin values can have clinical relevance in:

Diagnosing and tracking the treatment of sepsis 6, 2
Guiding antibiotic therapy and focusing treatment 2
Evaluating the severity of systemic inflammation and assessing the risk of mortality 4