Testing for antimicrobial susceptibility with deter- mination of the MIC of the antibiotic drug for the organism is recommended in choosing the optimal therapy for IE (Class I; Level of Evidence B). 2. Although not routinely recommended, determina- tion of the minimum bactericidal concentration of the antimicrobial agent chosen for treatment against the infecting organism may be considered in selected circumstances, such as with atypical organ- isms, organisms resistant to first-line antibiotic drugs, and unexplained failure to control bactere- mia (Class IIb; Level of Evidence C). Table 5. Principal Pathogenic Bacterial Agents Series Organism Johnson et al81 HACEK N/A 5 4 N/A Enterococcus species N/A 7 4 N/A Values indicate percentage of patients in the series. HACEK indicates Haemophilus species, Aggregatibacter species, Cardiobacterium hominis, Eikenella corrodens, and Kingella species; and N/A, not applicable. Baltimore et al Infective Endocarditis in Childhood: 2015 Update 1497 3. Synergy studies, with a -lactam agent and an aminoglycoside, although not always available and somewhat controversial, may be reasonable in determining optimal therapy of enterococcal or penicillin-nonsusceptible streptococcal IE (Class IIb; Level of Evidence C). Other Supportive Miscellaneous Laboratory Tests A variety of other nonspecific laboratory findings may sup- port the diagnosis of IE in children. The anemia of IE may be hemolytic or may represent the anemia of chronic dis- ease. Chronic low-grade hemolysis also may be caused by a prosthetic valve in the absence of IE. Leukocytosis is not a consistent feature of IE, but immature forms may be pres- ent on peripheral blood smears. Acute IE is likely to mani- fest leukocytosis. Thrombocytopenia can occur, particularly in neonates with IE. Hypergammaglobulinemia and elevated acute-phase reactants (eg, erythrocyte sedimentation rate and C-reactive protein) are present in a large proportion of patients. 