A knowledge of infections prevailing in the geographical locality is
an essential guide to diagnosis, especially with imported infections,
as is a knowledge of where and how to find the relevant organism.
Enquiry should be made about contacts among family, friends and
workmates. Persons following certain occupations may be exposed
to infection, e.g. leptospirosis occurs in abattoir and farm
workers and anthrax in handlers of hides and bone meal.
Recent surgical history may give a clue as to the origin of abscess
as a cause of unexplained fever.
Residence or travel abroad raises the possibility of malaria, amoebic
abscess of the liver or other exotic disease. In many infections a
diagnosis can often be made on clinical features, e.g. measles or
chickenpox. In others, a diagnosis may require confirmation by
microbiological, immunological, haematological,
histopathological or radiological investigations, see the
information box.
METHODS USED TO DIAGNOSE INFECTION
| MICROBIOLOGICAL |
- A. Recognition of causative agent
- In stained or fresh preparations, usually a smear malaria in
blood slide Vibrio cholerae in stool, diphtheria in throat swab,
bacilli in urine, staphylococci in pus smear, Entamoeba in rectal
scrape, plague bacilli in bubo aspirate. schistosome ova in rectal
snip, rickettsia in rash aspirate', fungi in skin scrapings,
pneumococci in purulent sputum, spirochaetes in condylomata
(dark ground microscopy). leprosy bacilli and leishmania in
slit skin smear
- By electron microscopy: viruses in stool,
herpesviruses from skin
- By histology of biopsy specimen: acid-fast bacilli in
leprosy and tuberculosis, Pneumocystis in pneumonia,
hepatitis 8 in liver, rabies virus in brain"
- B. Culture of causative organism
-
From blood typhoid, brucellosis, Gram-negative septicaemia, pneumococcal
pneumonia, HIV
- From bone marrow: tuberculosis, brucellosis, leishmaniasis,
histoplasmosis
- From other body fluids, faeces or tissues: urinary tract infection,
bacillary dysentery, sputum in pneumonia, liver in tuberculosis
|
| IMMUNOLOGICAL |
- A. Detection of microbial antigen
- Meningococcal and pneumococcal disease (blood, cerebrospinal fluid,
sputum, urine)
- B. Detection of antibody of IgM class
- Toxoplasmosis, hepatitis A, rubella, parvovirus
- C. Demonstration of antibody
- Rising titre: typhoid, brucellosis, HIV infection
- Closely linked to clinical syndrome: amoebic abscess, visceral
leishmaniasis
- Screening for latent disease: syphilis, schistosomiasis,
trypanosomiasis cruzi
- D. Delayed hypersensitivity skin testing
- Tuberculosis, histoplasmosis, leishmaniasis
|
| NON-SPECIFIC |
- A. Tissue biopsy
- Characteristic histology: hepatitis, leprosy
- Suggestive histology: tuberculosis, toxoplasmosis
- B. Radiology
- Association of site and pattern with infection: lobar
pneumonia, renal tuberculosis, muscular cysticercosis
- C. Scanning
- Isotope: detection of abscess, osteomyelitis
(polymorph scan)
- Ultrasound: abscess, hydatid cyst
- Computed tomography (CT) or magnetic resonance
imaging (MRI): intracranial infection, visceral
abscesses, mediastinal lymph node enlargement
|
| * Usually performed using immunofluorescent staining. |