Diagnosis including myelocytes, metamyelocytes, and bands, as well as

Diagnosis

Chronic myeloid leukemia usually detected during normal
routine health check or blood test performed for other medical reasons. Full
blood count test is first screening test performed in haematology laboratory
for screening of any haematological disorder. A lot of time, the
CML diagnosis is incidental on clinical basis, but prior to starting treatment
laboratory studies to determine the presence of the Philadelphia chromosome
(Ph) or BCR-ABL fusion are performed. There are number of laboratory tests used
in the diagnosis of chronic myeloid leukaemia including full blood count, blood
smear, bone marrow aspiration and biopsy, cytogenetics analysis, fluorescence
in situ hybridization and polymerase chain reaction.

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The most striking feature of CML in full blood count
is highly raised white blood cell count with median count of 175×109 /L. Other
indices of full blood count may show a moderate normocytic normochromic anemia
with reduced haemoglobin concentration. The platelets count can be normal or high,
as well as slight increase in red cells. The reticulocyte count can be normal
or moderately high. The blood smear shows normocytic normochromic red
cells with some nucleated red cells. White cell shows left shift with stages of
granulocyte maturation including myelocytes, metamyelocytes, and bands, as
well as varying degrees of eosinophils and basophils (Figure
4).

Figure 4: Characteristic features of
CML in blood film including basophilic and granulocytosis with neutrophils and
immature granulocytes.

 

In
CML the predominant cells observed under microscope are myelocytes and
segmented neutrophil (Mckenzie). However,
blast cells can also be seen with some promyelocytes. Even though neutrophils appear normal on blood film but cytochemically score
low on test called leucocyte alkaline phosphate (LAP). The significant of this
test is that it helps to differentiate between a leukemoid reactions possibly
due to infection as well as from polycythaemia Vera in which LAP activity is
high.

There
is also an increase in number of eosinophil and basophil in the CML patients’
blood smears with moderate increase in monocytes (Egan and
Radich,2016). Increase in numbers of basophil is a common finding in the blood
smears of CML patients and more than 90% patients have eosinophilia. However, absolute
monocytosis is not a common finding on peripheral blood smears but some patients
who have p190 BCR-ABL fusion protein instead of p210 can have increased monocytosis
(Etten, 2017).
On occasion
some overlapping features of chronic myelomonocytic leukaemia and CML such as
monocytosis, micro megakaryocytes and myeloid dysplasia are found, which can be
differentiated by carrying out further tests to identify Ph chromosome(Mckenzie, ).

BM
aspiration
and cytogenetic analysis are essential tests for the diagnosis of CML.
Without these two tests, we are unable to tell if there is an increase in blast
cells or basophils that will shift the staging from chronic phase to accelerated
or blast phase. Furthermore, we will not be able to know the other chromosomal
abnormalities apart from Ph chromosomes (Cortes and Kantarjian, 2016.) Bone marrow reveals granulocytic hyperplasia with immature
granulocytes, a pattern similar observed in the peripheral smear under microscope.
The differential count of leucocytes in marrow is normally within the range and
aur rods can be seen in myeloblast during blast phase of disease, which is an unusual
finding in CML. However, erythropoiesis is normal with reduced number of normoblast.
 Etten
et al, (2017) states that in both peripheral blood smears and bone marrow biopsy
blast cells between 10-19% are considered diagnostic for accelerated phase of disease
whereas over 20% of blast cells are consistent with blast phase of the disease.

The cytogenetic
of CML provides crucial information for its diagnosis as well as prediction of
prognosis and treatment outcomes. The test provide the information about number
and structure of the chromosomes. A bone marrow sample is used for cytogenetic
test due to its requirement of dividing cells. Majority of BCR-ABL translocations are readily identified by conventional cytogenetics
(figure). However, in small number of cases which involve complex changes
that still result in formation of a BCR-ABL transcript but without any detectable Philadelphia chromosome.

 

Figure: Shows the
Chronic myeloid leukaemia chromosome translocation. The translocation results
in a slightly longer chromosome 9 (first arrow at 9) and a shorter chromosome 22
(second arrow at 22) known as Philadelphia (Ph) chromosome.

 

The cytogenetics test have both advantage and disadvantage. A big
advantage of cytogenetic test is its ability to detect other chromosomal
structural abnormalities that may indicate advance disease. The down side of
this test is it only visualised 20 cells and not suitable for disease monitoring
and progression analysis as compared with FISH and PCR.

As compared with cytogenetic testing the FISH
uses probes fluorescently labelled for detection BCR-ABL genetic material (figure).
FISH has advantage over conventional cytogenetic tests as it quick and can be
performed on bone marrow as well as peripheral blood sample. Furthermore, FISH
has superior detection capability for BCR-ABL translocation as compared with
cytogenetics (Swansbury,2013). However, the
disadvantage of this technique is that as probes used are specifically designed
for BCR-ABL translocation, therefore any other rearrangement that may be present
will not be detected by this method and may require conventional cytogenetic
test (Egan and Radich,2016).

 

 

 

 

Normal                                                  
                          b) BCR-ABL                                                                                    Adopted from:
Shah and Areci (2014)

Figure: a) normal cells, two red and two green signals shows normal
ABL
and
BCR
genes,
respectively. b) The BCR-ABL fusion is visualized through the fusion
of the red and green signals, which is detected as a yellow fluorescence.

 

PCR detection of BCR-ABL is most sensitive
method for diagnostic purpose of CML. It detects 1 CML cells per 105 cells. This
high sensitivity of PCR allows use of blood sample than bone marrow for
diagnosis and treatment monitoring. The PCR method is considered a backbone in
the clinical decision-making. Appropriate reverse and forward primers are
designed which specifically binds with BCR-ABL transcript and amplify them. Although
there is a significant heterogeneity among BCR/ABL breakage in CML disease, but
majority of patients exhibit clones where exon 1e14 or 1e13 of BCR fuse with
ABL exon 2e11 and resulting BCR-ABL transcript is detected by single test
reaction (EGAN and RADICH,2016).