CLINICAL LAB SCIENEC

CHAPTER 14: CLINICAL CHEMISTRY 379
should be combined with reagent grade water or the provided diluent and mixed well
before obtaining an initial reading from a spectrophotometer.
Storage of the reagent is possible for up to 10 days when stored at 2°C to 8°C, and for
1 day when stored at 20°C to 25°C (room temperature), providing that the reagent has
not expired. The expiry date is provided on the box in which the reagent was received.
The reagent as packaged will be clear and colorless to straw-colored. The additive
as packaged will also be clear and colorless. After mixing the two reagents, the
working reagent should also be clear and colorless. Discard the working reagent if it
is turbid, as this may suggest contamination has occurred.
The preferred specimen is fresh and unhemolyzed serum. The samples should be
collected in the usual manner for any other clinical test requiring serum. Separate
serum and plasma from red blood cells promptly to minimize hemolysis, although
slight hemolysis will not significantly affect results. However, if erythrocytes remain in
the sample and become hemolyzed, they contain approximately 10 times more intracellular
AST than normal serum. If plasma must be used, the recommended anticoagulants
are oxalate, citrate, EDTA, and heparin.
In addition to hemolysis, other interfering substances such as certain drugs
(including illicit drugs) may affect AST determinations. Interferences with the AST
procedure commonly occur when triglyceride levels exceed 600 mg/dL. Bilirubin
values above 14 mg/dL, a common occurrence in jaundiced persons and those with
severe liver damage, may also interfere with this method.
Most kits for AST determination are suitable for manual methods as well as semiautomated
and automated instruments. The instrument settings are provided through
a package insert. The AST procedure is based on a kinetic rate change over a period
of time (Figure 14-3).
Equipment and Supplies
1. Gloves, disposable paper towels, and disinfectant or other cleaning solution
2. Report form provided by your instructor
3. Working reagents prepared as directed and monitored for deterioration
4. Spectrophotometer or other analyzer capable of measuring absorbance at 340 nm
30
Kinetic rate reaction
25
20
15
10
5
0
0
5 10 15 20
Time (minutes)
FIGURE 14-3 Kinetic rate reaction predicts the total level
of analytes in a short time.
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