Arcanobacterium Haemolyticum Workup

Updated: Mar 14, 2022
  • Author: Nicole Ufkes; Chief Editor: William D James, MD  more...
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Workup

Laboratory Studies

A complete blood cell count can show a white blood cell count of 7,100-23,000 cells/µL, with polymorphonuclear leukocytosis and a mean level of 13,000 cells/µL. [5, 6]

Monospot and antistreptolysin O titers are negative (unless concomitant infection is present), and help to distinguish A. haemolyticum infection from infectious mononucleosis and scarlet fever.

Kits are available to identify Arcanobacterium haemolyticum, such as the bioMérieux ANI card and the bioMérieux Coryne strip (a miniaturized test strip format), but are infrequently used in clinical laboratories. [56]

The diagnosis is most often made by culturing the pharyngeal swab on 5% blood agar plates (preferably rabbit or human blood agar, although sheep blood agar works if read at 48 h) at 37°C with 5% carbon dioxide for 24-48 hours. One study suggests that a 48-hour incubation on trypticase soy agar with 5% horse blood in 5% carbon dioxide may be the best medium. [19] Because most throat cultures are plated on sheep blood agar and read at 24 hours, the slow-growing colonies of A. haemolyticum may be missed. [70] At 48 hours, the bacteria produce nonpigmented 1-mm diameter colonies surrounded by a 3- to 5-mm diameter zone of hemolysis. Transmitted light may reveal a small, dark dot at the center of each colony. [8] A 2016 study suggests that hemolytic differential identification using the cyclic adenosine monophosphate (cAMP) inhibition and reverse cAMP tests may be effective in differentiating A. haemolyticum from other gram-positive coryneform bacillus and β-hemolytic streptococci. [23] These tests should used in conjunction with biochemical property test kits, colony size, and morphology.

If the clinician suspects this diagnosis, the laboratory should be notified because special media, longer incubation times, and further investigations are required for isolation and identification.