A 58-year-old woman comes to the office because of a 3-day history of fever, productive cough, and shortness of breath. She has rheumatoid arthritis treated with an IL-6 receptor antagonist and methotrexate for the past 2 years. Her temperature is 38.9°C (102.0°F), pulse is 98/min, and respirations are 22/min. Pulse oximetry on room air shows an oxygen saturation of 92%. Physical examination shows decreased breath sounds and crackles over the right lower lung field. Laboratory studies show:
Leukocyte count: 16,500/μL (80% neutrophils)
C-reactive protein: 8 mg/L
Erythrocyte sedimentation rate: 45 mm/hr
Sputum Gram stain shows gram-positive cocci in chains, and culture grows Streptococcus pneumoniae. Chest x-ray shows a right lower lobe infiltrate.
Which of the following best explains why CRP is an unreliable marker of infection in this patient?