Background & objectives: Acute tonsillitis is a disease that can often be cured with medical treatment. However, complications may occur during this disease process. One of these complications is peritonsillar abscess. In recent years, biomarkers have been frequently used in the diagnosis of diseases. The aim of the study was to reveal whether peritonsillar abscess develops after acute tonsillitis, and acute tonsillitis can be differentiated using biomarkers and which biomarker has higher predictive value for this differentiation. Methods: The control group consisted of individuals who were operated for septoplasty in the otolaryngology clinic, and the acute tonsillitis group consisted of individuals diagnosed with acute tonsillitis in the same clinic. Both groups were statistically compared in terms of mean platelet volume (MPV), neutrophil to lymphocyte ratio (NLR), platelet to lymphocyte ratio (PLR), systemic immune inflammation index (SII), infection discrimination index (IDI), plateletcrit (PCT) and lymphocyte to monocyte ratio (LMR) biomarkers. Statistically, significant biomarker values were compared between the subgroups in the tonsillitis group of those who had only acute tonsillitis and those who had peritonsillar abscess due to acute tonsillitis. Receiver operating characteristics (ROC) curve analysis was performed on biomarkers for their ability to predict the presence of peritonsillar abscess. Results: When the individuals who had only acute tonsillitis and those who had acute tonsillitis with peritonsillar abscess were compared in terms of biomarkers, there was a statistically significant difference between the mean MPV, SII and PCT (P=0.010, 0.021, 0.023, respectively). ROC analysis was performed to calculate the sensitivity and specificity of MPV, PCT and SII for the diagnosis of acute tonsillitis with peritonsillar abscess (sensitivity-specificity for MPV 51.9–72.7%, for SII 94.2–32.7%, for PCT 71.2–50.9%, respectively). Interpretation & conclusions: MPV, SII and PCT biomarkers may be useful to help clinicians predict peritonsillar abscess due to acute tonsillitis.