A 32-year-old multigravida (G3P2) at 28 weeks of gestation attends her routine antenatal clinic visit. Vital signs are normal, and uterine height is 28 cm. During the visit, a random blood glucose is found to be 165 mg/dL. She denies polyuria, polydipsia, or gestational diabetes in her previous pregnancies. What is the most appropriate next step in management?