How does the ESR CRP and WBC count help in identifying disease activity or an infection in an SLE patient?
• The ESR may remain elevated even when the disease is controlled, usually owing to a persistent polyclonal gammopathy
• CRP usually does not rise greater than six times the upper limit of normal even during a disease flare unless there is a systemic vasculitis, significant serositis, or an associated infection
• An SLE patient with fever and an elevated CRP should have an infection aggressively sought out
• WBC can drop with SLE activity or with infection but is more likely with an SLE flare.
• WBC can be deceivingly “normal” in an infected SLE patient who usually has a low baseline WBC due to their SLE. Beware of the febrile SLE patient who “normalizes” their WBC count.
• WBC differential showing a “left shift” is concerning for infection.
• Complement levels often rise in the setting of infection and drop with a SLE flare
• An elevated procalcitonin level or lactate may be suggestive of a bacterial infection in a febrile SLE patient