Wedge Method
(“Push”)
This method is
commonly used in manual as well as in automated and semi-automated
environments. When the wedge method is
carried out properly, a sufficiently large area is available for microscopic
examination: this area shows all cells barely touching or separated from each
other (monolayer part). The parts of the film farthest away from the start will
be too thin (with morphological alterations as a result), whereas the part
proximal to the start of the push will be too thick for microscopy.
Automated
devices are capable of providing excellent quality blood films, usually with
greater consistency than those obtained by manual methods (Pawlick and Relopez, 2000). The main concern with
wedge preparations is the uneven distribution of different cell types
(Stiene-martin, 2004). Monocytes (and other large leukocytes) in
particular are pushed to the end of the spread film (the “feathered” edge) and
to the sides. This leads to a 5% to 10% underestimation of monocyte presence
compared with monoclonal antibody–based flow cytometry differential counts (upper
normal limit in proportional count is 11% in normal individuals, not 10%, as
measured by microscopy) (Lebeck et al.,
2007).
Materials Used:
·
Glass
or microscope slide
·
Spreader
·
Cotton
wool
·
Capillary
tube
Procedure:
·
Mix
the EDTA anticoagulated blood well, to ensure even distribution of cells.
·
Place
a small drop of blood in the centre line of a slide about 1cm from one end.
·
Without
delay, a spreader is placed in front of the drop at an angle of about 30degree
to the slide. The spreader is moved back to make contact with the blood whereby
the blood spreads along the contact line. With a steady movement of the hand,
the blood is spread along the slide.
·
The
slide is labeled with a lead pencil for easy identification (Monica, 2006).
Possible Interferences
Patient
conditions influencing the preparation and quality of blood films are: anemia,
polycythemia, cord blood, platelet clumping,
cold
agglutinins, anti–red blood cell antibodies in severe hemolytic anemia, severe
rouleaux formation, and blood specimens from newborns. Moreso, for anaemic
patients, the angle of the spreader should be help at about 40degrees (Dacie and Lewis, 2012).
PRECAUTIONS TO
TAKE WHILE MAKING A THIN FILM
· Do not have too
large a drop of blood sample
· Do not incline
the spreader at too great an angle, as the film will be too thick for satisfactory
microscopic examination.
· Ensure to use a
spreader that is slightly narrower than the glass slide to minimize
distribution effects.
· Do not use a
dirty slide
· Ensure your
slide is free of moisture to avoid lysis of the blood.
· A glass slide cleaned
with alcohol and wiped dry gives the best result (Thomson et al., 2010).
QUALITIES OF A
GOOD THIN FILM
·
The
film should not be too long or too short. It should occupy 2/3 of the slide.
·
The
thin film should not have ridge
·
A
good thin film must have a head, a body, a tail
·
It
should not be too wide
·
A
good thin film must have a smooth tail and a thin tail
·
A
good thin film must be free of holes.
·
All
film should be allowed to dry on a horizontal or smooth surface
·
The
film made should always be labeled to avoid error or misplacement.
·
Films
made which is to be kept for some time before examination should always be
fixed (Lewis, 2003).
HOLES MAY BE
CREATED IN A THIN FILM IF:
·
The
slide is not grease free
·
The
sample is anaemic
·
The
sample has fat
·
Shaking
of hands usually affect cell distribution(Lewis, 2003).