Pediatric Haemophilus Influenzae Infection Workup

Updated: Dec 14, 2021
  • Author: Mobeen H Rathore, MD, CPE, FAAP, FIDSA; Chief Editor: Russell W Steele, MD  more...
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Laboratory Studies


This is the most important laboratory study in the context of suspected Haemophilus influenzae disease. In children, the organism causing these infections is bloodborne: hence, blood culturing is important in all cases. H influenzae can be cultured from samples of CSF, synovial fluid, pleural and pericardial fluid, and leading-edge aspirates of cellulitis.

Antigen detection

Numerous methods are available for identifying the H influenzae type b (Hib) PRP capsular polysaccharide antigen in clinical specimens. Suitable specimens for study may be obtained from urine and CSF. These are particularly helpful in the patient who has been pretreated with antimicrobial therapy. Antigen detection has little use in clinical practice, except in the situation mentioned above; most clinical laboratories do not offer this test.

Biochemical identification

Biochemical identification of H influenzae is based on the demonstration that growth in rich media (blood agar) is dependent on supplements, namely, factors X and V. Factor X is a heat-stable iron-containing protoporphyrin (hemin) that is essential for the function of enzymes in the electron-transport chain in aerobic metabolism. Factor V is the heat-labile coenzyme nicotinamide adenine dinucleotide (NAD).

Although both factors are present in erythrocytes, factor V must be released from the cell to sustain its growth; hence, standard blood agar is an unsatisfactory media for the growth of H influenzae. The lysis of RBC releases factor V, providing an exogenous source such as that in chocolate agar.

The metabolic requirement of factors X and V for growth remains the major basis for the laboratory identification of H influenzae. The growth requirements of H influenzae are fastidious, and the organism rapidly loses viability; therefore, clinical specimens must be handled expeditiously.

After overnight incubation, gray colonies appear; these have a diameter of 0.5-0.8 mm and are rough or granular. Encapsulated strains typically produce larger mucoid or glistening colonies.


Imaging Studies

Chest or lateral neck radiography, brain CT echocardiography, and technetium bone scanning may be appropriate.

Imaging studies depend on the clinical syndrome. In epiglottis, lateral neck radiography can be helpful if the clinical presentation is subtle, but the study should be performed cautiously, without undue delays, and a physician experienced in airway management should be present.

Approximately 50% of patients with pneumonia have evidence of pleural involvement at initial radiographic examination. Pneumonia can have a segmental, subsegmental, interstitial, or lobar pattern.



Procedures depend on the clinical circumstances. Necessary procedures may include the following:

  • Arthrocentesis

  • Pericardiocentesis

  • Endotracheal intubation or tracheostomy

  • Subdural tap

  • Leading-edge aspirate


Histologic Findings

H influenzae is a small gram-negative coccobacillus that may have considerable microscopic pleomorphism, which necessitates the careful and cautious interpretation of Gram stains of clinical specimens.