Among the most significant advances in late 20th century medical science are PCR and LC/MS, according to Paul Taylor, senior scientist in the department of clinical pharmacology at Princess Alexandra Hospital, Queensland, Australia. “Mass spectrometry is over 100 years old, yet it continues to exert a profound influence over the field of clinical diagnosis,” Taylor stated.
Mass spec with its speed, sensitivity, and specificity is an ideal tool to aid clinical diagnosis. With recent improvements in the technology, it is now accessible to clinical laboratories that lack a mass spec specialist. Historically limited to gas chromatography, the technology was confined to compounds that could be easily volatilized, so as to be amenable to ionization.
All this changed in the 1980s when electrospray and atmospheric pressure chemical ionization enabled the coupling of HPLC and mass spectrometry. Now, a wide range of analytes, all the way from small molecules to proteins such as hemoglobin, can be subjected to analysis. This resulted in an explosion of publications applying electrospray to a variety of macromolecular characterization problems.
“With strong advances in instrumentation driven by the pharmaceutical industry, we now possess an exquisite clinical analytical instrument in the LC-tandem mass spectrometer,” Taylor said. Among the most notable applications is newborn screening for inborn errors of metabolism such as phenylketonuria. While these tests were classically performed using bacterial auxotrophs, “because the automated mass spectrometer recognizes other aminoacidemias simultaneously, it represents a useful development toward a broad-spectrum neonatal screening method.”
LC/MS-based screening for inherited metabolic disorders has grown into a major clinical segment, with four million infants screened in 2005. Today isotope-dilution mass spectrometry, which employs loop injection and allows the analysis of up to 1,000 samples per day, is favored. The technology makes possible the coverage of rare disorders, which was not possible in the past. Another important application of LC/MS is therapeutic drug monitoring, especially immunosuppressive, antiepileptic, antipsychotic, and anti-AIDS drugs.
Yet another key clinical protocol is hormone measurement. Taylor discussed testosterone measurement by immunoassay, which has long been problematic due to the large range of concentrations in patients (e.g., children, females, and males) and because of issues of selectivity, accuracy, and sensitivity. LC/MS offers a viable alternative to measure this class of compounds.
Despite the merits of mass spec clinical diagnostic platforms, their acceptance has been limited. According to Taylor, a triple quadrupole instrument requires an initial capital investment of greater than $150,000, whereas immunoassays are supplied by the manufacturer on a reagent-rental basis. Furthermore, the complexity of mass-spec technology has been a daunting barrier in the past, although this is gradually being overcome as more user-friendly instruments come on-line.
LC/MS is an important technology, clearly more versatile than GC/MS, still evolving but possessing the potential to greatly extend the analytical capabilities of the clinical laboratory. Taylor predicted that new MS-relevant biomarkers for disease will be discovered, which will extend the limited range of the current technology.
“A quantum leap is required to help place LC/MS in the same league as the chemistry auto-analyzer,” Taylor said, “and we may require the introduction of a radical alternative technology such as MALDI or the combination of antibody-based sample preparation with mass spectrometry to move the process forward.”
Taylor concluded with a warning that there are many challenges facing the implementation of LC/MS in the clinical laboratory; the need for suitable internal standards, better sample preparation, improved automation, and the challenges of extensive interpatient variability. “But there is no doubt that this technology is making an impact on the diagnosis and treatment of patients now and will be increasingly relevant in the future,” he added.
Parathyroid Hormone Assay
Ravinder J. Singh, Ph.D., co-director of the endocrine laboratory at the Mayo Clinic, related a complex story on the building of an accurate serum assay for human parathyroid hormone (PTH). This critical hormone increases the concentration of calcium in the blood by interacting with its receptor. “Its measurement is essential for the diagnosis of a number of pathological conditions, including hypercalcemia, hypoparathyroidism, hyperparathyroidism, and renal osteodystrophy,” he stated.
Traditionally, parathyroid hormone levels are measured using immunoassays in which antibodies against the entire 84 amino acid molecule are employed. A truncated (7–84) peptide is an indicator of renal bone disease. Accurate quantitation of hormone levels is critical, given that elevated levels are frequently treated by surgery to remove the parathyroid.
Inaccurate measurement of parathyroid hormone levels has led to extremely serious consequences, according to Dr. Singh. In 2009, Quest Diagnostics was ordered to pay $302 million to address accusations that Nichols Institute Diagnostics, its subsidiary, sold misbranded test kits.
The case, targeting Nichol’s Advantage Chemiluminescence Intact Parathyroid Hormone Immunoassay, originated with a whistle-blower suit brought by Thomas Cantor. He noted that the test had an upward drift resulting in erroneously high values for serum parathyroid hormone levels, resulting in unnecessary parathyroidectomies and futile treatment with vitamin D analogues in misdiagnosed patients.
Dr. Singh and his colleagues have investigated the causes of the drift, and while there are numerous possible factors contributing to the failure of the assay, the most significant cause appears to be the reference calibrator, a human parathyroid hormone preparation from 1981. Today, none of the current PTH immunoassays use this calibrator.
With the help of LC/MS/MS to obtain accurate quantification of the 1–84 peptide, Dr. Singh’s team investigated cleanup methods and the properties of the peptides from the digestion of the complete hormone molecule. They were able to obtain information on the interference of the various PTH peptides running the entire length of the molecule. The group performed a study comparing the LC/MS/MS methods with Roche’s Cobas immunoassays.
“From these investigations we found that there is a need to harmonize immunoassays for quantifying intact PTH,” Dr. Singh stated. For this reason we have developed a reference method to specifically quantify the bio-active form of the complete, biologically active parathyroid hormone.”