What is the most likely cause of a rapidly rising markedly elevated C‑reactive protein in a 74‑year‑old former smoker with a several‑month history of fever that resolved on admission, normal white blood cell count and differential, low procalcitonin, and benign prostatic hyperplasia with normal PSA?

Malignancy (particularly occult solid tumor) is the most likely cause

In a 74-year-old former smoker with rapidly rising CRP (38→138 mg/L over 3 days), prolonged fever that resolved, normal WBC/differential, and low procalcitonin (0.05), you should aggressively pursue occult malignancy as the primary diagnosis.

Clinical Reasoning

Why Malignancy is Most Likely

The clinical pattern here is highly characteristic of paraneoplastic inflammation:

• Procalcitonin <0.05 essentially excludes bacterial infection 1. In hospitalized patients with markedly elevated CRP, bacterial infections typically show median CRP of 120 mg/L with elevated procalcitonin, while solid tumors show median CRP of 46 mg/L with low procalcitonin 2.

• The rapid rise (38→138 mg/L in 3 days) with normal WBC argues against infection 3. When febrile patients have elevated CRP but normal WBC counts, 82% have infection, but these patients typically show clinical signs of acute illness and elevated procalcitonin 3.

• Prolonged fever (several months) that spontaneously resolved is classic for paraneoplastic fever, not acute infection 4. Paraneoplastic fever is significantly more frequent in metastatic disease 4.

• Former smoking history at age 74 dramatically increases risk for lung cancer, which commonly presents with elevated CRP and constitutional symptoms 5.

What This is NOT

• Not acute bacterial infection: Procalcitonin 0.05 is too low. Bacterial infections causing CRP >100 mg/L show procalcitonin elevation and clinical toxicity 1.

• Not chronic inflammatory disease: While rheumatologic diseases can elevate CRP, they represent only 7.5% of markedly elevated CRP cases and only 5.6% of CRP >250 mg/L 1. The rapid 3-day rise is atypical for chronic inflammation.

• Not smoking-related CRP elevation alone: While smoking raises baseline CRP (mean 2.05 mg/L in current smokers) 56, it does not cause CRP >100 mg/L or rapid escalation.

Immediate Diagnostic Workup

Order these studies immediately:

1. CT chest/abdomen/pelvis with contrast - screen for lung, GI, renal, pancreatic malignancies
2. Age-appropriate cancer screening if not current:
   • Colonoscopy (given age, smoking history)
   • PSA already normal (excludes prostate cancer)
3. Basic labs: LDH, liver enzymes (AST/ALT to exclude hepatic pathology), albumin
4. Consider PET-CT if initial CT unrevealing, given high suspicion for occult malignancy

Critical Pitfalls to Avoid

• Do not assume resolved fever means resolved pathology. Paraneoplastic fever is intermittent and does not require ongoing fever for diagnosis 4.

• Do not wait for WBC abnormalities. In one study, 3.8% of febrile ED patients had elevated CRP with normal WBC, and 82% had infection, but in cancer patients specifically, paraneoplastic fever commonly presents this way 34.

• Do not attribute this to BPH. Normal PSA excludes prostate cancer, and BPH does not cause systemic inflammation of this magnitude 78.

Prognosis

Mortality in patients with markedly elevated CRP is 8.6% overall but 37% in those with malignancy 1. The combination of advanced age, smoking history, and this inflammatory pattern carries significant mortality risk, making urgent diagnosis imperative.