A 26-year-old woman presents with pain in the wrist and ankle joints for 6 months and a facial rash upon exposure to sunlight since summer. Exam: alopecia, irregular, circular skin lesions on the face and neck with slightly raised edges, multiple superficial ulcers in the oral cavity, liver palpable 0.5 cm below the costal margin, spleen just palpable below the costal margin. Lab: positive LE cells. According to revised ACR criteria, she meets the diagnosis of systemic lupus erythematosus (SLE). Which of the following statements about her laboratory tests is INCORRECT?