Diagnostic Odysseys: The Critical Role of Viral Transport Media

Doctor holding a test blood sample tube with positive marburg virus.

The successful diagnosis of viral infections relies heavily on proper collection, transport, and timely inoculation of clinical specimens containing infectious viral particles. Different viruses exhibit wide variability in composition, structure, morphology, stability and susceptibility to inactivation. Optimized transport systems and conditions are essential to maintain viral infectivity during shipment to the testing laboratory. A 1990 article reviewed viral transport media and methods from published literature.

The goal of transport media is to stabilize pH, provide protective proteins, control osmolality, and restrict contamination of specimens. Liquid media are commonly used with swab collections that contain high viral titers. Swab materials like calcium alginate can inactivate certain viruses, so cotton, rayon, Dacron or polyester are safer choices. Swab-tube systems like Culturette and Virocult effectively transport viruses like herpes simplex virus (HSV) from clinical samples, achieving isolation rates around 50%. These tubes contain liquid media into which specimen can be dispersed from the swab before culture inoculation.

Various liquid transport media have been formulated, providing salts, buffers, proteins and antibiotics. Formulations include cell culture medium, Hanks balanced salt solution (BSS), hypertonic sucrose, skim milk, bacteriological broths, and bentonite clay. Cell culture medium preserves viruses when holding times are short before culture inoculation. BSS-based media with added albumin or gelatin are widely used. Hypertonic 40-45% sucrose excellently stabilizes labile viruses like respiratory syncytial virus (RSV). However, high sucrose is diluted before culture to prevent toxicity. Broth media including veal infusion, tryptic soy and tryptose phosphate recover diverse viruses from clinical material. Bentonite clay also recovers many viruses while controlling contamination, but can hinder observing cytopathic effects. Newer liquid media avoid cell culture inhibitors like charcoal and agar. Stabilizing proteins include albumin, gelatin and skim milk.

Refrigerated transport at 2-8°C is optimal, although ambient shipment succeeds for most viruses using proper media. However, temperature extremes or freezing should be avoided, apart from -70°C ultracold storage. Maximum viral titer and infectivity occur within 3-7 days after symptom onset, but viruses remain isolatable after transport times up to 12 days in optimized media. Transport regulations dictate triple-packing with absorbent material and insulated shipping container. Viruses are also concentrated from large-volume water samples by adsorbing onto filters for transport and elution in the lab. Studies show sucrose solutions excel at stabilizing labile viruses like RSV and cytomegalovirus (CMV). Refrigeration best preserves viruses, which may lose infectivity after freezing at -20°C, while -70°C storage optimally maintains viability. Cryoprotectants like sucrose, DMSO and proteins protect against freezing damage.

In summary, optimized viral transport methods enable successful isolation and diagnosis of infections caused by viruses including HSV, varicella-zoster, adenovirus, reovirus, enterovirus, influenza, RSV and many others. Using proper media and conditions, viral infectivity can be maintained to permit testing at distant reference laboratories. Appropriate viral transport systems facilitate laboratory diagnosis of viruses with diverse structural stability.

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