The Narrow Path -My Daily Devotionals: July 2007

Hi guys!

IN case you STILL don't know, I'm currently working in the groundbreaking biomedical industry! Its like a dream come true for me since I have aspired to be a medical technologist since sec 4. And it has finally happened! A 20 week attachment stint with the biggest hospital in Singapore (I'm not mentioning names here for confidentiality issues), which seems to be providing me a guaranteed opening for a biomedical career!

Working in the clinical lab is really exciting because it is the busiest place around! (especially in a 24h routine lab where I work in.) Here, we do, general tests like basic chemistry tests (Na, K, Cl) and renal, liver, and thyroid function tests, as well as more specialized ones like tumor markers, hormones and even haemotological tests like ferritin and serum folate/ B12.

In the clinical lab, machines are man's best friends. All we have to do is sort out the samples, label and send them to their respective stations, and the people there will load the samples, press some buttons and unload after the machine has finished with the sample processing. In my workplace, patient blood specimens are stored for 3 days, after which they are discarded under strict ISO14k EMS guidlines (read up LMQA ISO14k notes). Sounds simple but thats just a brief overview of the workflow.

Immunoassays are used as the diagnostic method in most, if not all, clinical lab machines. Thus it is fundamental to understand this important principle.

Immunoassay Formats

Assays can be either heterogenous or homogenous.

Heterogenous assays require separation from a bound and unbound label and use chemoluminescence (measured by a photomultiplier in the analyzer) as the measurement method.

Homogenous assays, on the other hand, does not require separation (washing step), and uses photometry (measured by a spectrophotometer) as a common measurement method. Homogeneous assay requires the light-producing reaction to be affected in some way by binding of the ligand to its receptor. Antibody binding leads to a change in the signal measurable from a label or matter in competition for binding first the substance being measured.

So.. Which is better??

Homogeneous assays are desirable because of their simple mix-and-measure nature. This can be achieved when the key binding event (mix) modulates signal generation (measure) without the need for separating bound and unbound ligands.

However, it is far simpler to design heterogeneous assays in which labeled binding pair complexes are separated from unbound labeled reactants. Most chemiluminescent reactions can be adapted to this assay format, by labeling either with a chemiluminescent compound or with an enzyme and using a chemiluminescent substrate. Most commercially developed immunoassays are of this type.

Diagrams (click to enlarge)
Homogenous Assay of Thyroxine