Sepsis is a severe, heterogeneous and potentially lethal disease. Different clinical manifestations depend mainly on the etiological agent, spread of the infection, but most of all, on functioning of the patient’s immune system. Cancer, malnutrition, autoimmune diseases, diabetes and organ transplantation are primary risk factors for sepsis. The separate, but very important risk factors, are invasive diagnostic and therapeutic procedures (1, 2). Early diagnosis is the key point for effective treatment. Difficulty in early diagnosis of the infection and evaluation of its severity are very often caused by lack of evident clinical symptoms. Symptoms may be present only at advanced stage of the disease. This applies to patients who have many factors that stimulate immune response. High mortality in severe infections is directly related to delayed introduction of proper treatment. Regardless of the therapeutic option used, the main factor influencing successfulness of the therapy is time from diagnosis to implementation of the treatment. Determining a marker that organism release at early stage of the infection, would enable to introduce proper treatment at the initial stadium. Additionally, early intervention would probably reduce mortality (3).

One of the first biomarkers of the infection, applied in 1921, is erythrocyte sedimentation rate (ESR). ESR has also been used in many other medical conditions since that time. Although ESR is still in use, today it plays only minor role in diagnosing infection (1).

178 biomarkers has been detected and described in over 3000 studies in recent years (2). The basic characteristics of each biomarker that is going to be used in clinical practice include high sensitivity and specificity. Additional advantages are low incidence of false positive and negative results.

This protein is produced mainly in the liver and adipose tissue and is described as an acute phase protein. Normal serum level in healthy population is not higher than 7-10 mg/l (4). Test sensitivity and specificity are 30-100% (5). CRP level is elevated in infections, autoimmune diseases, massive trauma and cancer diseases. Maximal elevation is observed 1-3 days after damaging insult, depending on the disease. Its level normalizes after 7-10 days of successful treatment (4). Observations in many centers show that CRP is not a good infectious marker in newborn and preterm infants. CRP elevation is delayed in comparison to other acute phase markers (procalcitonin, α-1 antitrypsin, inhibitors and pro-inflammatory cytokines). Therefore it is recommended to use several markers in this age group (4). Determination of CRP level has a greater diagnostic value than white blood count or ESR. However, etiology of the infection can not be determined basing of the CRP serum level (5). Clinical studies by Soetino et al., published in 1989, showed that determination of CRP level in the cerebro-spinal fluid might be useful in differentiation between viral and bacterial meningitis. CRP level was estimated in the cerebro-spinal fluid taken from children with bacterial and viral meningitis. C-reactive protein was detected only in patients with bacterial meningitis. It was not present in viral meningitis. Test sensitivity in the purulent infection was 91% comparing to 46% of the microbiological testing (gram stain of the CSF smear) (6). CRP is commonly used as a monitoring tool of the clinical status. It is a diagnostic and prognostic factor in patients with acute pancreatitis, sepsis, meningitis and active inflammatory process. Decrease in CRP level is an indirect indicator of effectiveness of the antibiotic therapy (4).

Procalcitonin has been strongly considered as a marker of inflammation, infection, sepsis and septic shock in the last decade. This tendency is reflected by multifold publications that describe increasing number of multicenter clinical studies and some meta-analysis. Procalcitonin is a peptide composed of 116 aminoacids with molecular weight of 13 kDa. Physiologically it is a precursor of calcitonin and is synthetised in C cells (neuroendocrine cells) in the thyroid gland. In experimental studies procalcitonin was also isolated from adrenal glands, spine, lungs, adipose tissues and gastrointestinal tract. PCT itself does not show any hormonal activity. Its physiological role has not been fully explained. PCT probably takes part in the intercelullar cAMP production, regulates interactions between integrins and influences monocyte chemotaxis. The normal serum level in healthy population is below 0.1 ng/ml. PCT level may be helpful in determining clinical stage of the infection. PCT is eliminated by the renal system (3, 7, 8). Its level increases 2-4 hours after infectious insult to the tissues. The maximal elevation is detected after 6-8 hours. PCT is a relative sensitive and specific factor for the bacterial infection (74-100% and 70-100%, respectively) (5, 9). Studies that compare WBC, CRP and PCT levels indicate that PCT is an early infection marker, which precedes CRP elevation at least for 24 hours. It applies to both – organ and systemic infections (3). Kim et al. estimated role of PCT and CRP as early infection markers in patients with neutropenic fever with suspected infection. PCT and CRP levels were statistically analyzed. Diagrams of ROC and AUC were higher for PCT than CRP. The greater the AUC, the greater test diagnostic power, which has optimal sensitivity and specificity parameters. Authors suggest that procalcitonin is more effective in early diagnosis of the infection in patients with neutropenic fever (10). PCT might be a marker of severe infections, in which etiological factor can not be directly detected and clinical symptoms are unspecific (11). PCT elevation in response to infection occurs prior to increase in CRP, WBC and ESR levels. This plays a crucial role in infections in small children. Though, it should be noticed that elevated PCT levels in newborn maintain for 48 hours after birth (4). Diagnosing infection in children with urinary tract anomalies allows for early initiation of antibiotics and PCT monitoring shortens therapy duration (4, 12). PCT determination is recommended in differentiating between bacterial and viral meningitis. PCT elevation in bacterial meningitis leads to early introduction of the proper antimicrobial therapy. Many studies have not detected PCT in the cerebrospinal fluid (4, 13). These observations are true not only in respect to children. Many studies of the youngest population of patients is concentrated on children with neutropenia, because infection in this group is a negative prognostic factor. Determination of the PCT level enables to differentiate between bacterial and viral infection and initiation of the proper treatment (14). PCT determination is not only limited to differentiation between infectious and noninfectious etiology, but also plays an important role in treatment monitoring and elimination of most serious complications. Nobre et al. analyzed duration of antibiotic therapy in patients with sepsis (15). Patients with abdominal infection were included to the study. Antibiotic was stopped when PCT level decreased for about 90% of the primary value (not until 3 days if PCT was < 1.0 ng/ml or 5 days if PCT level was >1 ng/ml). In the control group antibiotic was given for 9 days (maximal length was 33 days). Therapy duration was reduced by 3.5 days in the study group. Mortality was similar in both groups, but duration of hospitalization was reduced in the study group. These data show that controlling PCT level is useful in evaluation of antibiotic efficacy (15). Multicenter studies concerning antibiotic efficacy in the ICU patients confirmed that therapy duration had been reduced by 27-37% depending on the clinical diagnosis (16). Monitoring of PCT level probably allows for detection of the infection at early stage, when antibiotic should be introduced regardless of the surgical procedure (17-19). Chromik et al. determined PCT level after colorectal surgery within 3 days after operation (17). Patients were given conventional perioperative antibiotic prophylaxis. PCT elevation > 1.5 ng/ml was an indication for using 3^rd generation cephalosporin in the first group of patients. Patients in the second group were given antibiotic after developing clinical symptoms. Early antibiotic introduction in the first group decreased number of severe infectious complications comparing to the 2^nd group (18, 20). PCT elevation correlates well with a secondary peritonitis and subsequent multiorgan failure (21). Therefore PCT evaluation leads to early introduction of antibiotic and surgical intervention. PCT elevation after major abdominal surgery is helpful in diagnosing complications in early postoperative period. Elevation of CRP and IL-6 level were not that significant (22). Charles et al. showed that PCT level is significantly higher in cases of sepsis caused by Gram-negative comparing to Gram-positive bacteria (23). PCT levels in developing sepsis are much higher than in patients already presenting symptoms of the infection (24). PCT levels are much higher in sepsis caused by pneumonia than in pneumonia without symptoms of sepsis. In addition, PCT elevation precedes CRP elevation for at least 24 hours (10, 25). All these data show that PCT determination is helpful in diagnosing and monitoring antibiotic therapy in intensive care unit, especially in septic patients (26).