Clinical Chemistry

Cobas is an analyzer used in biochemistry. Some of the common tests include Troponin-T, CKMB, HbA1c, and total PSA etc. The tests may use one of the two principles, competition or sandwich.

An example of a test that uses the competition principle is cortisol. Firstly, the sample will be incubated with a cortisol specific biotinylated antibody and a ruthenium complex labelled cortisol derivative. Depending on the concentration of cortisol in the sample and formation of respective immune complex, the biotinylated antibody binding site will be occupied partly by cortisol from the sample and the ruthenium complex labelled cortisol derivative.

Next, streptavidin coated microparticles are added. The mixture undergoes a second incubation whereby the immune complexes bind to the solid phase through interaction between the biotin and streptavidin.

Subsequently, the mixture will be added into the measuring cell. The microparticles will be magnetically attracted onto the surface of the electrode. Unbound substances are removed with Procell. Voltage will be applied onto the electrode and induce chemiluminescent emission that will be measured by a photomultiplier.

An example of test that uses sandwich principle is CKMB. The difference would be the use of two antibodies; CKMB specific antibody labelled with ruthenium complex and biotinylated anti-CKMB antibody. Therefore, during the first incubation, it would form a sandwich complex whereby the CKMB present in the sample will occupy the binding site of the CKMB specific antibody and the biotinylated anti-CKMB antibody will bind to the CKMB specific antibody. The subsequent process that takes place is similar to that mentioned above.

The clinical significance of cortisol is for diagnosis of Cushing's syndrome (overproduction of cortisol) and Addison's disease (underproduction of cortisol). The reference range of cortisol is 5-25ug/dL at 8am and 2-17 ug/dL at 4pm. This is due to significant diurnal variation in cortisol levels, whereby it is highest in the morning and lowest at night.

The clinical significance of CKMB is for diagnosis of myocardial ischaemia, e.g. in acute myocardial infarction and myocarditis. However, CKMB may also be present in stroke and rhabdomyolysis. Therefore, total CK and Troponin-T levels should also be tested to differentiate between the two clinical implications. In addition, CKMB will be present 3-8 hours after onset of cardiac symptoms. Therefore, sensitivity of CKMB test is dependent on the time the sample was collected. The reference range of CKMB is 0-5ng/mL.

Liyana
0703827F