Specimens of pus or wound discharge collected on swabs are difficult to evaluate macroscopically, particularly when the swab is immersed in a transport medium. Specimens of pus, received in a syringe or in a sterile container, should be evaluated carefully by an experienced technician for colour, consistency, and odour.
The colour of pus varies from green-yellow to brown-red. A red colour is generally due to admixture with blood or haemoglobin. The aspirate from a primary amoebic liver abscess has a gelatinous consistency and a dark brown to yellowish brown colour. Pus from postoperative or traumatic wounds (bums) may be stained blue-green by the pyocyanin pigment produced by Pseudomonas aeruginosa.
The consistency of pus may vary from a turbid liquid to one that is very thick and sticky. Exudates, aspirated from a joint, the pleural cavity, the pericardial sac, or the peritoneal cavity, are generally liquid, with all possible gradations between a serous exudate and frank pus.
Pus originating from a draining sinus tract in the neck should be inspected for small yellow “sulfur” granules, which are colonies of the filamentous Actinomyces israelii. The presence of sulfur granules suggests a diagnosis of cervicofacial actinomycosis. Small granules of different colours (white, black, red, or brown) are typical of mycetoma, a granulomatous tumour, generally involving the lower extremities (e.g., madura foot), and characterized by multiple abscesses and draining sinuses. The coloured granules correspond to either filamentous bacteria or fungal mycelium.
Pus from tuberculous “cold abscesses” (with few signs of inflammation) is sometimes compared with soft cheese and called “caseum” or “caseous pus”.
A foul feculent odour is one of the most characteristic features of an anaerobic or a mixed aerobic - anaerobic infection, although it may be lacking in some instances. The odour, together with the result of the Gram-stained smear, should be reported at once to the clinician as it may be helpful in the empirical selection of an appropriate antibiotic. It will also help in determining whether anaerobic cultures are needed.