The adoptive mother of a 5-month-old child born in another country brings the child to the nurse practitioner's office. The child's medical history is unknown. The mother states that over the past 2 days, the child has been experiencing frequent bouts of crying that are unrelieved by holding. The mother reports that the child's hands and feet are slightly swollen during these episodes. On examination, the child's temperature is 101°F with mild splenomegaly. Sickle cell disease is suspected. This diagnosis may be confirmed by: