Molecular diagnostics: the best is yet to come.

‘In other words, if an adenine forms one member of a pair, on either chain, then on these assumptions the other member must be thymine; similarly for guanine and cytosine…If only specific pairs of bases can be formed, it follows that…the sequence on the other chain is automatically determined’ [1xMolecular structure of nucleic acids. Watson, J.D. and Crick, F.H.C. Nature. 1953; 171: 737Crossref | PubMed | Scopus (5811)See all References][1]. It's been almost fifty years since these words were written by Francis Crick and Jim Watson. By the 1980s, this notion of molecular hybridization was being exploited for diagnostic applications in lymphoproliferative disorders. Other seminal work allowed restriction fragment length polymorphism analysis [2xEnzymatic amplification of β-globin genomic sequences and restriction site analysis for diagnosis of sickle cell anemia. Saiki, R.K. et al. Science. 1985; 230: 1350–1354Crossref | PubMed | Scopus (5765)See all References][2] to complement the highly useful but labor-intensive Southern blot [3xDetection of specific sequences among DNA fragments separated by gel electrophoresis. Southern, E.M. J. Mol. Biol. 1975; 98: 503–517Crossref | PubMed | Scopus (20555)See all References][3].The invention of the polymerase chain reaction (PCR) in the mid 1980s gave birth to the notion that by then, so-called molecular diagnostics could eventually become a routine element of the arsenal of the clinical diagnostics laboratory usually housed in the hospital pathology department. 15 years have passed since then. Where are we today?There is a modest list of United States Food and Drug Administration (FDA)-approved molecular diagnostic tests [www.ampweb.org (click on ‘Resources’ and then the link for ‘FDA-Approved Molecular Diagnostics Tests’)]. Many more tests are performed expertly by clinical molecular diagnostics laboratories as so-called ‘home-brew’ tests but with rare exception (e.g. cystic fibrosis mutation detection), these are usually low-volume tests. The vast amount of press in professional and lay journals should not fool anyone into thinking that laboratory medicine today is ‘molecularized.’ The overwhelming majority of the diagnostic laboratory assay menu comprises conventional testing in hematology, clinical chemistry, microbiology and serology.In keeping with the title of this journal, however, and the topics covered in this issue, one must of course realize that although the ‘minority status’ of molecular diagnostics is a truism, at the same time, the trend in human disease diagnostics is clearly moving in the molecular direction. In the molecular pathology laboratory I directed, we signed out five test results in 1991. By the time I left seven years later, we were signing out nearly 15 000 per year and the rate of increase continues at 15–20% per year. Now, some reference laboratories are signing out 1800 molecularly-based cystic fibrosis tests per week.The reasons for this rapid increase are, in my opinion, two-fold. First, the era of genomics has provided us with the sequences of important organisms including Homo sapiens and many bacterial and viral pathogens. The number of diagnostically relevant targets identified is growing. Paraphrasing Clare Fraser of The Institute for Genomic Research (Rockville, MD, USA) in her presentation at the Association for Molecular Pathology Meeting (Philadelphia, PA, 17 November 2001), genomics has provided us with a parts list without the instruction manual. In this context, genomics paves the way for the upstream search for diagnostics, vaccines and therapeutics, so that the endgame can be realized sooner. The second reason for the continuing growth in molecular diagnostic testing volume is highly related to the first reason of the discovery of diagnostically relevant targets. As our understanding of the molecular basis of inherited, somatic and infectious (including drug resistance/sensitivity) disease increases, molecular diagnostics is starting to play a key role in patient management: for diagnostic tests to be useful (read: ordered by physicians), they must make a significant difference in patient management.If one accepts these premises, then it is only a matter of time before molecular diagnostics becomes normalized into the mainstream diagnostic pathology laboratory. Initially, molecular diagnostics laboratories were centralized because the technology was so labor-intensive and specialized. As molecular diagnostic professionals proliferated and molecular techniques became applicable across a broader variety of laboratory specialities, decentralization began. In time, molecular diagnostics will be broadly applicable in laboratory medicine and test volumes will rival those of established specialities like hematology and clinical chemistry. In vitro diagnostics companies will rise to the challenge of making molecular analysis as routine as conventional automated laboratory testing. Much is made of the concept of DNA microarrays. The genomics of complex diseases and the need to analyze panels of infectious agents to make the differential diagnosis on a patient are but two examples of why the need for arrays in tomorrow's diagnostic pathology laboratory is an inevitability.Arguably one of the most medically important molecular diagnostic tests – HIV genotyping – recently received FDA clearance. This test, although semi-automated, is gel electrophoresis-based. The field will trend away from this sort of assay and move towards automated readouts of real-time PCR detectors downloaded to laboratory information systems. Molecular diagnosis of complex disorders will, in time, become as diagnostically routine as single-gene disorders. Testing for minimal residual neoplastic disease will increase. Panels for multiple drug-resistant pathogens will be developed, used and exploited to control outbreaks. My guess is that all of these predictions will ring true by 2010, perhaps sooner. More distant, but no less real, is the prospect of gene-expression profiling in disease management. Because the patterns, associations and fundamental observations are still being made and because the bioinformatics systems and interpretation algorithms are largely immature [4xMinimum information about a microarray experiment (MIAME)-toward standards for microarray data. Brazma, A et al. Nat. Gen. 2001; 29: 365–371Crossref | PubMed | Scopus (2814)See all References][4], use of gene expression in the routine diagnostic setting is at least a decade away. 1000 or 10 000 spot gene expression arrays won't be in use unless software renders all those data transparent and the physician gets a final piece of advice following an algorithm established by laboratory medical professionals (pathologists and clinical scientists). The promise of gene expression profiling, as of all molecular diagnostics, remains great, however, simply because it is likely to improve patient management. Truly, the best is yet to come.