The definitive diagnosis of enteric fever requires the isolation of S. typhi or S. paratyphi from blood, bone marrow or other sterile sites, rose spots, stool or intestinal secretions. Other than a positive culture, no specific laboratory test is diagnostic for enteric fever.
The definitive diagnosis of enteric fever requires the isolation of S. typhi or S. paratyphi from blood, bone marrow or other sterile sites, rose spots, stool or intestinal secretions. Other than a positive culture, no specific laboratory test is diagnostic for enteric fever.
Harrison’s Principles of Internal Medicine
Blood Culture:
This is the gold standard investigation for Enteric Fever. Positive in only 40-60% cases, due to high rate of antibiotic use in endemic areas and small number of S. tyhpi organism typically present in the blood. ( i.e < 15/ mL)
Bone Marrow Culture:
Most sensitive. (85-90%) and its yield is not reduced by upto 5 days of prior antibiotic therapy, unlike blood culture.
Urine and Stool Culture:
Positive after first week.
Stool culture is negative in the first week in 60-70% of the cases.
Culture of intestinal secretions:
Culture of intestinal secretions, best obtained by a noninvasive duodenal string test, can be positive despite a negative bone marrow culture.
If blood, bone marrow, intestinal secretions all are cultured, the yield is > 90%.
Complete Blood Count:
Leucopenia, Neutropenia in 15-25% cases.
However, Leucocytosis is more common among children during first 10 days of infection, may even reach upto 20,000 – 25,000/mm3.
Thrombocytopenia is the marker of severe illness and DIC.
Widal Test:
Done after first week for the detection of salmonella antibodies in the patients serum.
Simple and rapid test, but there is limited sensitivity and specificity, especially in endemic regions.
Components of Widal Test:
- TO
- TH
- AO
- AH
- BO
- BH
Interpretation of Widal Test:
- No single test is diagnostic, it may be presumptive.
- Increased TO Recent Active infection by any serotype. TO > 1:160 is suggestive of infection. However, a rising titre over a period of 7-10 days is, however, of greater value.
– raised TO + raised TH: S. typhi
– raised TO + raised AH: S. paratyphi A
– raised TO + raised BH: S. paratyphi B
There is some sharing of the O antigen between Salmonella typhi and the paratyphoid organisms, but there is no sharing of the H antigen. If, then, O antibody is present to both typhoid and paratyphoid A and B organisms, the cause may be active infection.
- Only increased TH: Anamnestic Reaction.
Those individuals, who had suffered from enteric fever in the past, sometimes develop anti-Salmonella antibodies during an unrelated or closely related infection. This is termed anamnestic response and can be differentiated from true infection by lack of any rise in titre on repetition after a week. but if H antibody is present to all three, the to cause can only be previous vaccination.
- Raised TH, AH, BH: Past vaccination.
When H and O antibodies are produced in a patient’s serum, whether by a natural infection or by vaccination of T.A.B., they remain at a high level for about six months, after which the O antibodies fall rapidly, but the H may remain at a high level for years. Clearly, then, in interpreting a Widal test, a high H titre is of much less significance than a high O. Elevation of antibody titre against O antigen has a better diagnostic value.
If an inoculation response is present, a repeat test in a few days time may help, but, if the initial titres are high there is often no rise in the second test, even in the presence of active infection (Mol, 1948). The H antibody is said to rise in response to non specific stimuli, the anamnestic reaction. This may be so, but a rise in H titre during afebrile illness is much more likely to be due to typhoid fever.
- The Vi agglutination test is of some help in detecting carriers. A titre of 1/5 is said to indicate the presence of Salmonella typhi in the body. But at least 25% of carriers have negative Vi tests, and a large number of normal people have positive Vi titres. In an unselected population, the number of Vi reactors who are non-carriers is much higher than the number who are carriers. In an area where typhoid is common, the Vi test is likely to be of little use in detecting carriers. In Britain, provided one is working in a narrow field of suspects, the test is more useful, though even then, it will detect only three out of every four carriers. The question is fully discussed in several surveys.