Objective <p>Familial Mediterranean Fever (FMF) is a monogenic autoinflammatory disease characterized by recurrent febrile attacks and serositis. Despite its clinical severity, especially in pediatric patients, there is no specific biomarker to objectively differentiate between attack and remission periods. This study aimed to evaluate the clinical utility of immature granulocyte percentage (IG%) in predicting FMF attacks and compare it with traditional inflammatory markers.</p> Methods <p>Ninety-six pediatric FMF patients diagnosed according to Tel-Hashomer criteria and 68 age- and sex-matched healthy controls were included. IG%, neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), systemic immune-inflammation index (SII), and conventional acute phase reactants (CRP, SAA, fibrinogen) were assessed during both attack and attack-free periods. ROC analysis was performed to determine the diagnostic performance of IG%.</p> Results <p>IG%, NLR, PLR, and SII were significantly elevated during attacks compared to both attack-free patients and controls (<i>p</i> &lt; 0.001 for all). IG% showed no significant difference between attack-free patients and controls (<i>p</i> = 0.581), indicating its specificity for acute inflammation. IG% correlated with SAA (<i>r</i> = 0.250, <i>p</i> = 0.014) and platelet count (<i>r</i> = 0.222, <i>p</i> = 0.030). ROC analysis identified an IG% cut-off value of 0.3 with 81.3% sensitivity and 85.4% specificity (AUC = 0.891). For comparison, CRP exhibited the highest diagnostic accuracy (AUC = 0.981), followed by SAA (AUC = 0.963). Although slightly less powerful than these conventional markers, IG% offers unique clinical value due to its rapid availability and cost-effectiveness. IG% levels were unaffected by MEFV mutation subtype.</p> Conclusion <p>IG% is a reliable, rapid, and cost-effective biomarker for distinguishing acute attacks in pediatric FMF. While CRP and SAA demonstrated higher overall diagnostic accuracy, IG% provided greater specificity by normalizing during remission, highlighting its role as a complementary marker. Its independence from genetic variations and availability via routine CBC supports its practical use in clinical monitoring.</p> <p><Table Float="No" ID="Taba"> <tgroup cols="2"> <colspec align="left" colname="c1" colnum="1" /> <colspec align="left" colname="c2" colnum="2" /> <tbody> <row> <entry align="left" nameend="c2" namest="c1"> <p><b>Key Points</b></p> <p>• <i>Immature granulocyte percentage (IG%) is a simple biomarker that can be automatically obtained from routine complete blood counts without additional cost or laboratory procedures</i>.</p> <p>• <i>In pediatric Familial Mediterranean Fever (FMF), IG% was significantly elevated during acute attacks but not in attack-free periods, suggesting its specificity for acute inflammation</i>.</p> <p>• <i>ROC analysis demonstrated good diagnostic performance of IG% (AUC = 0.889), while CRP and SAA showed higher AUCs but remained mildly elevated during remission, reducing their specificity</i>.</p> <p>• <i>Compared to conventional markers, IG% provides a rapid, cost-effective, and practical advantage for clinicians, especially in pediatric settings</i>.</p> <p>• <i>IG% may serve as a complementary biomarker in FMF management and has potential applicability across other acute inflammatory conditions</i>.</p> </entry> </row> </tbody> </tgroup> </Table></p>