A 20-year-old student presents with dry cough, low grade fever and diffuse interstitial infiltrates on chest radiograph. Serum shows autoagglutination at room temperature that reverses on warming to 37 degrees Celsius. A titre of 1:128 is obtained against human O group red blood cells. This finding is most consistent with infection by:

• A Mycoplasma pneumoniae ✓
• B Streptococcus pneumoniae
• C Coxiella burnetii
• D Chlamydia psittaci

Explanation

Cold agglutinins are IgM antibodies directed against the I antigen of human erythrocytes, seen in about half of patients with Mycoplasma pneumoniae primary atypical pneumonia. They agglutinate red cells below 37 degrees Celsius and dissociate on warming, and a titre of 1:64 or more, or a fourfold rise, supports the diagnosis. Streptococcus pneumoniae causes lobar consolidation with sputum findings rather than this pattern, so it is the key distractor killed by the reversible cold-reactive agglutination.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.