Biomedical Engineering Reference
In-Depth Information
In the early years of LC-MS/MS application in clinical laboratories, chromato-
graphic separation was looked upon as rather unnecessary with tandem mass spec-
trometers being understood as extremely selective measuring devices. Thus, many
LC-MS/MS methods with minimal degree of chromatographic resolution and ana-
lyte retention times close to the void time of the chromatographic systems (“dilute
and shoot” approaches) have been described. However, from the issues discussed so
far, the requirements of proper sample preparation and sufficient chromatographic
separation prior to MS/MS detection have become evident.
In this chapter we have so far reviewed issues which can introduce inaccuracy
into LC-MS/MS analyses and which are specifically related to this technology.
It must be noted, however, that LC-MS/MS analyses in clinical pharmacology at
present extensively involve manual handling steps which are prone to gross errors—
independent of the analytical technology used. Such gross handling errors include:
permutation of samples during sample preparation; permutation in the positioning
of sample vials in autosampler racks (positive sample identification is not realized
yet); incorrect transfer of data from report printouts to a LIMS (in most laboratories
no LIMS connection of the chromatography software is implemented). In general it
must be emphasized that the risk of gross errors is probably higher for LC-MS/MS
compared to analyses run on standard clinical chemistry analyzers with positive
sample identification from sample introduction to LIMS-transfer of the data.
Consequently, progressive automation is essential to improve the reliability of
LC-MS/MS analyses in clinical pharmacology.
Systematic error can also be introduced in the process of standardization; this is
in particular true for research analyses in pharmacological research: Calibration
materials and QC materials are typically prepared by spiking drug-free serum or
plasma samples with solutions of the target analyte to desired concentration levels.
The purity of drug compound specimens used for this process might be incorrect
(e.g. due to sample breakdown). Furthermore some compounds require a high con-
tent of organic solvent in the stock solution for complete dissolvation; during the
spiking of serum samples for the production of calibrator materials, the matrix of
the spiking solution can induce local protein precipitation with loss of the analyte
by co-precipitation. Such problem can be addressed by external quality assessment
based on reference preparations or proficiency testing schemes which, however, are
rarely available for innovative and research analytes.
3.4
Therapeutic Drug Monitoring: Immunosuppressants
A low therapeutic quotient, narrow therapeutic bandwidths, drastic side effects when
leaving these, massive drug interactions, high intra- and inter-individual variability in
metabolization, and the resulting loss of dosage-exposure relationship require tight
control of the monitoring of whole blood-drug levels in the therapy with cyclosporine
A, tacrolimus, sirolimus, everolimus. Particularly during the first few weeks following
transplantation finding the right dosage is often difficult. Measurements must be fast,
