PHOTOMETER-LINKED MICROCOMPUTER SYSTEM ENABLES PRECISE CHROMOGENIC PROTHROMBIN TIME ASSAYS

In coagulation diagnostics photometric assay procedures are in widespread use. Due to the availability of new specific chromogenic peptide substrates, automated instruments play an important role in clinical routine laboratory diagnosis. For research work, the benefit of expensive industrial photometric coagulation systems is questionable, especially as the program cannot be adapted by the user. For this purpose we developed an inexpensive microcomputer-controlled measuring system as well as a suitable photometric assay which allows to determine chromogenic clotting times with any conventional spectrophotometer.

Absorbance data were taken from the analog chart-recorder output of a double-beam spectrophotometer, digitized by a 12 bit analog-to-digital converter and read by the computer via an interface. Menu driven, user orientated / user dialogue based compiled BASIC software was written to enable data acquisition and processing.

During the chromogenic assay procedure, automatically collected absorbance data were displayed, stored, analyzed immediately, saved on disk for later kinetic analysis and printed.

Preliminary results with our chromogenic PT-assay indicate excellent reproducabi1ity of the test. The clotting time itself is defined as the interval from the beginning of the test to the moment when a preselected absorbance change occurred. Standard curves can automatically be calculated by regression routines after measurement of reference values.

It must be emphasized that the occurence of fibrinogen-generated turbidity during the chromogenic assay sometimes influences the total absorbance significantly. For this reason the reaction time has to be limited by a low optical endpoint setting.