Molecular Diagnosis

Conventional diagnostic tools — microscopic examination, serum analysis, urine analysis — are indirect and often not specific enough to catch a disease early, because a pathogen usually needs to reach a high concentration in the body before it causes visible symptoms. Molecular diagnostic techniques instead detect the pathogen's own molecules directly, at far lower concentrations, well before symptoms appear. ELISA (Enzyme Linked Immunosorbent Assay), discovered by Eva Engvall and Peter Perlmann in 1971, detects specific antibodies or antigens in a sample such as serum or urine: the suspected antigen is immobilised on a plate, a specific antibody is added and allowed to bind it, an enzyme-linked anti-antibody is then added, unreacted material is washed away, and the enzyme's substrate is added last, producing a coloured product whose intensity is proportional to how much antigen is present. It comes in four main forms — direct, indirect, sandwich and competitive ELISA — is highly sensitive (able to detect antigen at nanogram levels), needs no radioactive isotopes, and is the standard test for determining HIV status as well as for measuring hormones such as human chorionic gonadotropin. PCR (Polymerase Chain Reaction), developed by Kary Mullis in 1983 (Nobel Prize 1993), instead amplifies a chosen stretch of DNA into billions of identical copies through three repeated temperature-controlled steps: denaturation (heating to about 95°C splits the double-stranded target DNA into single strands), annealing/renaturation (rapid cooling lets short primers bind the flanking sequences on each strand), and extension/synthesis (heating to about 75°C lets heat-stable Taq DNA polymerase extend each primer along its template). Because Taq polymerase survives the repeated high-temperature denaturation step, the whole cycle can be repeated dozens of times without adding fresh enzyme each round, doubling the target DNA with every cycle. A variant, RT-PCR, first converts an RNA template (via reverse transcriptase) into cDNA so that RNA viruses or gene-expression levels can be amplified and studied the same way. Clinically, PCR's sensitivity makes it invaluable: it can identify a pathogen's DNA directly from a clinical sample (blood, sputum, spinal fluid) without needing to culture the …