Laboratory diagnosis of Enteric fever

Enteric fever, also known as typhoid fever is an acute illness associated with fever caused by the Salmonella typhi bacteria. It can also be caused by Salmonella paratyphi, a related bacterium that usually causes a less severe illness (also known as paratyphoid fever)

The disease is transmitted by faecal-oral route – via contaminated water and food.

LABORATORY DIAGNOSIS

Specimen collection:

The best specimen for diagnosis is blood for culture examination. The Serum sample is also very useful for detection of antibodies during 2^nd & 3^rd week of disease.

Following samples may be required for culture examination in certain conditions

·        Urine (positive in 2^nd and 3^rd week ,25% positivity)

·        Faeces (Valuable in patient on Antibiotic)

·        Bone-marrow (Recommended in patient having high suspicious of enteric fever / PUO especially when blood culture is negative)

·        Bile

Occasionally the organisms can be isolated from Pus / CSF / Gall bladder / Mesenteric lymph nodes

 

Laboratory investigations:

There is no role of direct microscopic examination from the specimen.

Following are the common investigations to be carried out:

1.     Blood Culture – The gold standard for diagnosis of typhoid fever

 

Volume of blood to be collected:

5-10 ml for Adult&1-3 ml for children

 

The culture media used:

Hartley’s broth

Trypticase soy broth

 

-      Check for turbidity and evidence of growth after 1, 2, 3 and 7 days if bottles showing signs of growth – Do culture on solid media in Mac Conkey agar and Blood agar

On day 7, all the bottles sub-cultured before being discarded as negative

-      If Automated system is used it gives indication for negative or positive.

If system indicates positive, subculture from the bottle; & if system indicates negative, discard the bottle and report it as negative.

2.     Culture of other specimen:

Following culture media are used

-      enrichment media:  Salanite F or tetrathionate broth &

-      Selective media i.e DCA or Wilson - blair Media.

 

3.     Serological test:

 

a.     Widal test

      Widal  test should be done after ONE WEEK of onset of fever.

      Principle: It detects antibody & not bacteria or antigen.

b.     TyphiDot Rapid assay (detect antibody)

c.      IgM dipstick test • Detects IgM antibodies in serum and whole blood

d.     Enterocheck - WB • Immunochromatographic test in cassette

 

4.     Other Non specific Tests:

• Moderate anemic, increased ESR, thrombocytopenia, lymphopenia

• Slightly elevated PT and APTT, decreased fibrinogen level

• Liver transaminases & bilirubin – increased 2 times than normal

• Mild hyponatremia & hypokalemia

 • Serum ALT: LDH > 9:1 = viral hepatitis < 9:1 = typhoid hepatitis

5.     Molecular methods • PCR is sensitive, but not widely used

 

DETECTION OF CARRIERS

Detection of salmonella carriers can be done in the following ways:

FEACAL carriers: can be detected by isolation of bacilli from bile or feces. Repeated sampling increase chances of isolation.

Urinary carriers: can be detected by repeated urine culture.

Detection of Vi agglutinin indicate recent carrier.

1.     List the investigations used for diagnosis of typhoid fever with reference to its significance according to duration of diseases.

2.     Draw labelled diagram of Tube widal test