Page 368 - Textbook of Pathology, 6th Edition

352 Laboratory Findings

The diagnosis of IM is made by characteristic haemato-
logic and serologic findings.
1. HAEMATOLOGIC FINDINGS. Major abnormalities
in blood are as under:
i) TLC: There is a moderate rise in total white cell count
(10,000-20,000/μl) during 2nd to 3rd week after infection.
ii) DLC: There is an absolute lymphocytosis. The lympho-
cytosis is due to rise in normal as well as atypical T
lymphocytes. There is relative neutropenia.
iii) Atypical T cells: Out of the total lymphocytes in the
peripheral blood, essential to the diagnosis of IM is the
presence of at least 10-12% atypical T cells (or mononucleosis
cells) (Fig. 14.9). The mononucleosis cells are variable in
appearance and are classed as Downey type I, II and III,
of which Downey type I are found most frequently. These
atypical T lymphocytes are usually of the size of large Figure 14.9 Peripheral blood film showing atypical lymphocytes in
lymphocytes (12-16 μm diameter). The nucleus, rather infectious mononucleosis.
SECTION II
than the usual round configuration, is oval, kidney-shaped
or slightly lobate due to indentation of nuclear membrane
and contains relatively fine chromatin without nucleoli, a) Specific antibody against EBV capsid antigen show elevated
suggesting an immature pattern but short of leukaemic titers in over 90% cases during acute infection. IgM class
features. The cytoplasm is more abundant, basophilic and antibody appears early and is thus most useful for
finely granular and may contain vacuoles. The greatest diagnosis of acute infection. IgG class antibody appears
number of atypical lymphocytes is found between 7th to later and persists throughout life; thus it does not have
10th day of the illness and these cells may persist in the diagnostic value but is instead used for assessing the past
blood for up to 2 months. exposure to EBV infection.
iv) CD 4+ and CD8+ T cell counts. There is reversal of b) Antibodies against EBV nuclear antigen are detected 3 to
CD4+/CD8+ T cell ratio. There is marked decrease in 6 weeks after infection and, like IgG class antibodies,
CD4+ T cells while there is substantial rise in CD8+ T cells. persist throughout life.
v) Platelets. There is generally thrombocytopenia in the c) Antibodies to early antigens may be elevated but are less
first 4 weeks of illness. useful for making diagnosis of IM. However, titers of these
antibodies remain elevated for 3 to 6 months and their
2. SEROLOGIC DIAGNOSIS. The second characteristic levels are high in cases of nasopharyngeal carcinoma and
laboratory finding is the demonstration of antibodies in African Burkitt’s lymphoma.
the serum of infected patient. These are as under:
d) IgA antibodies to EBV antigen are seen in patients of
i) Test for heterophile antibodies. Heterophile antibody nasopharyngeal carcinoma or those who are at high risk
test (Paul-Bunnell test) is used for making the diagnosis of developing this EBV-induced cancer.
of IM. In this test, patient’s serum is absorbed with guinea iii) EBV antigen detection. Detection of EBV DNA or
Haematology and Lymphoreticular Tissues
pig kidney. Serum dilutions are prepared which are used proteins can be done in blood or CSF by PCR method.
for agglutination of red cells of sheep, horse or cow and
are reported as heterophile titer of test serum. A high 3. LIVER FUNCTION TESTS. In addition, abnormalities
serum titer of 40 or more times is diagnostic of acute IM of the liver function test are found in about 90% of cases.
infection in symptomatic case in the first week. These include elevated serum levels of transaminases
Heterophile antibodies peak during the 3rd week in 80- (SGOT and SGPT), rise in serum alkaline phosphatase and
90% cases. The test remains positive for about 3 months mild elevation of serum bilirubin.
after the illness started. Thus, the test has to be repeatedly
performed. Similar antibody is also produced in patients LEUKAEMOID REACTIONS
suffering from serum sickness and has to be distinguished Leukaemoid reaction is defined as a reactive excessive leuco-
by differential absorption studies. Heterophile antibodies cytosis in the peripheral blood resembling that of leukaemia
are not demonstrable in children under 5 years of age or in a subject who does not have leukaemia. In spite of
in quite elderly. Currently, more sensitive and rapid kit- confusing blood picture, the clinical features of leukaemia
based test for heterophile antibodies, monospot, is also such as splenomegaly, lymphadenopathy and haemorrhages
available. are usually absent and the features of underlying disorder
ii) EBV-specific antibody tests. Specific antibodies against causing the leukaemoid reaction are generally obvious.
the viral capsid and nucleus of EBV can be demonstrated Leukaemoid reaction may be myeloid or lymphoid; the
in patients who are negative for heterophile antibody test: former is much more common.