(3) The finger should be thoroughly dry prior to puncture; blood will not well
up on a finger that is moist. Furthermore, the alcohol or other antiseptic used can
coagulate the blood proteins causing cell clumping and erroneous values as well as
dilute cell volumes. This will result in incorrect counts and differentials.
(4) Finger punctures should be made along the lateral aspect of the
fingertip. More nerve endings are located on the fingerprint area of the fingers;
therefore, more pain results from punctures in this area. Scars can also form in these
sensitive areas, and difficulty may be encountered in puncturing a callous. All of these
difficulties are eliminated by drawing the blood from the lateral rather than the ventral
aspect of the finger.
a. Anticoagulants are used to prevent the clotting of the blood specimens and
the reagent employed should not bring about alteration of blood components.
Unfortunately, many anticoagulants can alter cell structures as well as coagulation. The
anticoagulants most often used are ethylene-diamine-tetra-acetate (EDTA), ammonium-
potassium oxalate (Heller and Paul double oxalate), and heparin.
b. The choice of anticoagulant will depend on the analysis to be made.
Ethylene-diamine-tetra-acetate (EDTA) is the anticoagulant of choice for most
hematological analyses. This anticoagulant causes a minimum of distortion to the cells
and platelets. It does not dissolve quickly in blood, however, so the tube must be
inverted four or five times after blood is added. The dipotassium salt is prepared as a 1
percent solution in distilled water, and a final concentration of 0.5 ml of anticoagulant for
each 5 ml blood is used. Another common anticoagulant is arnmonium-potassium
oxalate (Heller and Paul double oxalate). This combination of oxalates does not shrink
or enlarge the red blood cells appreciably. It is essential, however, to add an optimal
volume of blood to the oxalate, no less than 3.5 nor more than 6.0 mI.
c. Heparin does not alter the size of cellular components. It is, in fact, the
standard for comparison of anticoagulant distortion. Heparin is more expensive and
dissolves less readily than double oxalate salts. Approximately 0.5 to 1.0 mg is required
to anticoagulate 5 ml of blood for 72 hours. The quantity of anticoagulant noted above
in each case is sufficient to prevent clotting of the blood specimen. On the other hand,
an excess of anticoagulant should be avoided because too much will result in distortion
of cells and hemolysis. Ideally, differential blood smears should not be prepared from
blood that contains an anticoagulant.
d. If oxalate is added to vials and dried in an oven, take great care to avoid
temperatures above 80oC. Oxalates are converted to carbonates by prolonged
exposure to elevated temperatures. Under normal circumstances, it should not be
necessary to prepare your own oxalate solutions since prepared anticoagulant vacuum
tubes are available from Federal medical supply sources.