The purpose of this study is to evaluate the use of a novel method to measure adherence to immunosuppressant medications in predicting rejection episodes in children who had a liver transplant.

During the course of their illness as many as 50% of children who had a liver transplant stop taking their medications. Non-adherence is the most important reason for organ rejection in long term survivors of pediatric liver transplantation. In order to address this important risk-factor effectively, the first step is to evaluate a method that would identify non-adherence in these children. Medication blood levels that are obtained as a part of clinical practice in transplant centers can be used to determine whether the patient is adherent or not. This multi-center observational study tests the ability of an objective measure of adherence to immunosuppressant medications that involves the use of routinely obtained tacrolimus blood levels to predict organ rejection in children who had a liver transplant.

Inclusion Criteria:

• The patient is between > 1 year of age and less than 18 years of age at enrollment
• Guardian's consent, child assent (in accordance with each institution's IRB policies).
• The patient is prescribed tacrolimus (either brand or generic formulation).
• The patient has been seen in the enrolling center's clinic at least once in the last two years.

Exclusion Criteria:

• The patient received a liver transplant less than 1 year prior to enrollment.
• The patient has had more than one solid organ transplant (including marrow replacement).
• The patient has had biopsy-proven rejection within the past six months.
• The patient has been diagnosed with Hepatitis C.
• The guardian or child (in a developmentally-appropriate manner) do not understand the study procedures. This will be verified by asking both guardian and child (if 6 years old or older) to repeat the study procedures.
• The patient is only seen for consultation - most or all of the child's routine care is provided at another center (or in a community clinic).
• Either the patient or the guardian is actively psychotic or severely disoriented due to any cause, including hepatic encephalopathy (temporary exclusion) or severely mentally retarded as defined in the Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV).
• The patient is not medically stable or is hospitalized.
• The treating physician has instructed the participant not to obtain tacrolimus levels for at least one year.

Pediatric patients receiving liver transplantation