10-20 mcg/mL Critical value: >20.0 mcg/mL

Criterion: recovery plus or minus 0.8 ug/mL of initial value at concentrations less than 8 ug/mL or plus or minus 10% of initial value at concentrations greater than 8 ug/mL. Icterus: No significant interference from bilirubin up to an I index of 60 (approximate bilirubin concentration: 60 mg/dL). Hemolysis: No significant interference from hemoglobin up to an H index of 1450 (approximate hemoglobin concentration: 1000 mg/dL). Lipemia (Intralipid): No significant interference up to an L index of 333 (approximate triglycerides concentration: 2000 mg/dL). There is poor correlation between turbidity and triglycerides concentration. No significant interference from total protein up to 12 g/dL or rheumatoid factor up to 180 IU/mL. Due to cross-reactivity to 1,3 dimethyluric acid, this assay should not be used to quantitate samples from uremic patients (1-2). The incidence of patients with antibodies to E. coli beta-galactosidase is extremely low. However, some samples containing such antibodies can result in artificially high theophylline results that do not fit the clinical profile. As with any assay employing mouse antibodies, the possibility exists for interference by human anti-mouse antibodies (HAMA) in the sample, which could cause falsely elevated results. References: (1) Breiner R, McComb, Lewis S and Wong SHY, Marzouk N and Kapke GF. Positive interference with immunoassay of theophylline in serum of uremics, letters to the editor. Clin Chem. 1985;31:1575-1577. (2) Nelson KM, Mathews SE, Bowers LD. Theophylline concentrations may be falsely high in serum of uremic patients, letters to the editors. Clin Chem. 1983;29:2125-2126.