Precise Infliximab Exposure and Pharmacodynamic Control

Inclusion Criteria: 1. Written informed consent from the patient (≥18 years old) or from parent/legal guardian if patient is \<18 years old 2. Written informed assent from patient when age appropriate 3. Diagnosis of Crohn's disease within the last 90 days (luminal-only or luminal with a perianal fistula or abscess treated with antibiotics for at least 7 days) 4. ≥6 years to ≤22 years of age, anti-TNF naïve and starting infliximab 5. Clinical activity and luminal inflammation, defined by both (1) and (2) * (1) PCDAI≥10 (\<18 years old) or CDAI ≥150 (≥18 years old) in last 60 days before the decision to start infliximab * (2) SES-CD\>6, or SES-CD\>3 for isolated ileal disease (or a report of large intestinal ulcerations)\* within the last 60 days or a fecal calprotectin \>250 μg/g within last 75 days prior to screening 6. C-reactive protein \>1.0 mg/dL in last 30 days and/or fecal calprotectin \>250 μg/g within last 75 days prior to screening 7. Negative TB (tuberculosis) interferon-gamma release test and a negative urine pregnancy test for female patients (if menstruation has started) Exclusion Criteria: 1. Diagnosis of ulcerative colitis or inflammatory bowel disease-unspecified 2. Prior use of anti-TNF therapy (infliximab, adalimumab, certolizumab pegol, or golimumab) 3. Internal (abdominal/pelvic) penetrating fistula(e) in last 180 days 4. Intra-abdominal abscess/phlegmon/inflammatory mass in the last 180 days 5. Active perianal abscess (receiving oral antibiotics for \<7 days) 6. Intestinal stricture (luminal narrowing with pre-stenotic dilation \>3 cm) and surgery planned in the next 90 days 7. Have tested positive for Clostridium difficile toxin (stool assay) or other intestinal pathogens within 14 days of screening unless a repeat examination is negative and there are no signs of ongoing infection with that pathogen. 8. Current hospitalization for complications of severe Crohn's disease 9. Planned use of methotrexate or 6-mercaptopurine (azathioprine) during the induction (first 3 doses of infliximab) phase 10. Current ileostomy, colostomy, ileoanal pouch, and/or previous extensive small bowel resection (\>35 cm) or any CD surgery planned within the next 90 days 11. History of autoimmune hepatitis, primary sclerosing cholangitis, thyroiditis, or juvenile idiopathic arthritis 12. Treatment with another investigational drug in the last four weeks 13. History of malignancy (including lymphoma or leukemia) 14. Currently receiving treatment for histoplasmosis 15. History of TB, human immunodeficiency virus (HIV), an immunodeficiency syndrome, a central nervous system demyelinating disease, history of heart failure or receiving intravenous antibiotics in last 14 days for any infection 16. Currently pregnant, breast feeding or plans to become pregnant in the next 1 year 17. Inability or failure to provide informed assent/consent 18. Any developmental disabilities that would impede providing assent/consent