Page 656 - Clinical Hematology_ Theory _ Procedures ( PDFDrive )

640 PART 8 ■ Fundamentals of Hematological Analysis

T ese include activated partial thromboplastin time (AP ), signal. An ampli ed signal is converted to a digital value or

prothrombin time (P ), and actor assays. urther processing. T e computer-processed results are sub-

A new generation o P point o care testing is the sequently sent to the visual display monitor and printer.

HEMOSENSE INRatio2 IN/P (Milpitas, CA). T is hand- A system is ready or operation when the temperature

held instrument provides the P and corresponding inter- indicator reads 37 ± 1°C. Preprogrammed modes select the

national normalized ratio (INR) value by measuring the test parameters or each test method, which determines the

electrical impedance using resh capillary whole blood. T e proper volumes o specimen and reagents. T e appropriate

INR system per orms a modi ed version o the one-stage P amounts o reagents are placed in the speci c reagent stor-

test. T e clot ormed in the reaction is detected as a change age wells. Pressing the start button initiates the test cycle.

in the electrical impedance o the blood sample that occurs T e optical density (absorbance) o the reaction mixture is

when brinogen is converted into brin. T e test strip itsel then monitored until the rate o change exceeds a predeter-

consists o layers o transparent plastic, one o which is an mined level or a de ned period, indicating the presence o a

electrode layer (Fig. 30.24). brin clot end point. T e time (in seconds) o the end point

is stored and may be printed or displayed on demand at the

Photo-Optical Methods end o each series o determinations.

Quality control in these systems includes automatic sel -
T e principle o photo-optical measurement is that a change checking o the optical system and the storage o standard

in light transmission measured as optical density (absor- and assay curves. A coe cient o determination can be used

bance) versus time can be used to quantitatively determine to check precision.

the activity o various coagulation stages or actors. Photo- In addition to routine clot testing, newer instruments

optical clot detection systems can be used or the determina- o er chromogenic channel models to automate the growing

tion o a wide variety o assays (e.g., AP , P , brinogen range o specialized diagnostic tests in coagulation.

levels, and thrombin time). Quantitative actor assays based

on the AP ( actors VIII, IX, XI, and XII) and quantita-

tive actor assays based on the P ( actors V, VII, and X) are Viscosity-Based Detection System

examples o available assays. Viscosity is de ned as the resistance that a material has to a

T ese microprocessor-controlled instruments have sepa- change in its orm. I this principle is used as a mechanism or

rate detector cells with their own red LED light source, which clot detection, the natural thickening (viscosity) is monitored

is driven by a constant current regulator to give each a noise- by the motion (amplitude o an oscillating steel ball in a spe-

ree light beam. T e light beam passes through a cuvette, cially designed cuvette) as a change in orm takes place. T e

where it is altered by brin clot ormation. T e light beam nal result is accurate and is insensitive to colored plasma,

then passes through a di user and alls on the sensor, which lipemic plasmas, bilirubin, or turbid reagents and reliable

instantly converts the transmitted light into an electrical measurement or the hemostasis laboratory. Diagnostica

Stago manu actures an instrument based on this principle.

T e steps in the viscosity-based detection system (VDS)

are as ollows:

1. Movement o the steel ball is triggered by two activating

coils, working alternately to induce and maintain a natural

oscillation.

2. When the start reagent is added, the detection starts

immediately.

3. When the ball starts oscillating le and right, a chronom-

eter (clock) begins to time the clotting o the sample.

4. As the ball oscillates le and right, the amplitude or

motion is also measured. A peak is ormed when the ball

is detected in the center o cuvette.

5. Amplitude is monitored during the entire clotting process.

6. As the clot appears, the viscosity increases, and the ampli-

tude decreases.

7. Based on di erent algorithms, the chronometer is stopped

even i the clot is peak, and/or the ball is still in motion.

Platelet Agglutination

T e ristocetin co actor assay measures the ability o a patient’s

FIGURE 30.24 HEMOSENSE INRatio2 IN/P . (Courtesy o plasma to agglutinate ormalin- xed platelets in the presence

HEMOSENSE Milpitas, CA.) o ristocetin. T e rate o ristocetin-induced agglutination is