Representative nonexhaustive flowchart of testing that can be considered during the evaluation of a pediatric patient with suspected HLH. No one algorithm is right for every patient. Clinicians should always consult with local multidisciplinary specialists regarding evaluations and consider consultation with physicians with expertise in HLH. Genetic testing should also be performed in many cases. CBC, complete blood count; CMV, cytomegalovirus; HSV, herpes simplex virus; PCR, polymerase chain reaction; PET-CT, positron emission tomography-computed tomography; TB, tuberculosis; TREC, T-cell receptor excision circle; VZV, varicella-zoster virus.