CLINICAL LAB SCIENEC

CHAPTER 9: LABORATORY MATHEMATICS 227
plasma is to indicate the importance of standard conditions for the process to take
place as well as the extremely accurate measurement of the enzyme substrate of
18.75 millimoles. Many chemistry determinations utilize this method for accuracy,
speed, and specificity. The conditions and the determination of the factor
are presented below.
Assay Conditions
Chemistry reactions often must have specific conditions to work properly in a
reasonable amount of time or at all. When laboratory determinations are performed,
the conditions are engineered for accuracy, speed, and specificity. The
following set of conditions are required for most chemistry procedures and
are often programmed into the chemistry analyzers. Each reaction will have
some differing parameters, such as wavelength, test volumes, reaction time,
and temperature (Table 9-10).
Explanation of a Rate (Kinetic) Reaction
U/L = Delta (common term for changing absorbance) change per minute × Factor
Delta absorbance (Δ) is the change in absorbance as measured for a period of
time with use of the spectrophotometer. This is a rate (also called kinetic) reaction
for a specific substrate reacted on by a specific enzyme. Some medical tests
are reported in units per liter (U/L).
Clearance Tests
Some tests are developed to show how well certain body organs are removing
certain toxins and waste products that affect a patient’s health. The kidneys and
liver are the two organs that are most responsible for this task. The kidneys
reabsorb most of the components as they remove certain elements or excess
water. Some organic components are not reabsorbed to a great extent. One of
these is creatinine, a byproduct of energy conversion in the muscles. A creatinine
clearance test is extremely important in diagnosing certain kidney diseases,
and requires the analysis of both urine and blood creatinine. One other clinical
Table 9-10 Example of Assay Conditions for Basic Kinetic Chemistry
Reaction
The requirements for a test reaction called a rate or kinetic reaction are specifi c. Most procedures
will provide an assay sheet with the information necessary to manually calculate a value or to be
programmed into an autoanalyzer.
Wavelength 405 nm Temperature (constant) 37 ºC
Reagent volume 1 mL Sample volume 25 μL
Total test time 4 minutes (approx.) Millimolar absorptivity of substrate 8.5
Cuvette dimensions 1 cm (square) Units U/L
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