Differential Diagnosis for Patient with Rash while taking Ceftriaxone and Clindamycin

• Single most likely diagnosis:
  • Non-allergic rash to Ceftriaxone: This is likely because the patient developed a rash while taking Ceftriaxone and Clindamycin, but tolerated Cefazolin, suggesting the reaction might not be due to a cephalosporin allergy. The rash could be due to other factors such as a viral exanthem, a reaction to Clindamycin, or a non-allergic reaction to Ceftriaxone.
• Other Likely diagnoses:
  • Allergy to Clindamycin: The patient's rash could be an allergic reaction to Clindamycin rather than Ceftriaxone, as the patient tolerated Cefazolin, another cephalosporin antibiotic.
  • Drug interaction or side effect: The combination of Ceftriaxone and Clindamycin could be causing a drug interaction or side effect that manifests as a rash, rather than an allergic reaction to either drug individually.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.):
  • Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN): Although rare, these severe skin reactions can be life-threatening and require immediate medical attention. They can be caused by medications, including antibiotics like Ceftriaxone and Clindamycin.
  • Drug Rash with Eosinophilia and Systemic Symptoms (DRESS): This is a severe drug reaction that can be life-threatening. It is characterized by a rash, eosinophilia, and systemic symptoms, and can be caused by various medications, including antibiotics.
• Rare diagnoses:
  • Cephalosporin allergy with cross-reactivity: Although the patient tolerated Cefazolin, there is a small chance of a cephalosporin allergy with cross-reactivity to Ceftriaxone, which could cause the rash.
  • Serum sickness-like reaction: This is a rare reaction that can occur with certain medications, including antibiotics, and is characterized by a rash, fever, and arthralgias.