= Viral load
<Viral load> is the measured amount of a <virus> in a specified sample, commonly expressed relative to sample <volume>. The measured target and units must be stated. For <human immunodeficiency virus>, a familiar result is the number of viral <RNA> copies per millilitre of <blood>; <reverse transcription polymerase chain reaction> is one method used in viral-load testing.<Viral Load Test in the NIH HIV Glossary>[definition, RNA-copy units and example assay methods]{ref}
Normalization turns an assay count into a concentration. In a hypothetical calibrated measurement, 250 RNA copies attributed to $0.050\,\mathrm{mL}$ of the original specimen correspond to
$$
\frac{250}{0.050\,\mathrm{mL}}=5000\ \text{copies per millilitre}.
$$
The example assumes that extraction, dilution and assay calibration have already been accounted for. Counting a portion of an extract without that relationship would not establish the same sample-normalized value.
Ratios are often more informative than raw differences. A constructed decrease from 100,000 to 10,000 copies per millilitre is a tenfold reduction, a $90\%$ decrease, and a change of $-1$ in the base-ten <logarithm>. Those are three descriptions of the same numerical comparison, rather than three independent observations.
Measurement has limits. A <limit of detection> concerns the smallest signal or corresponding amount distinguishable with stated certainty by a procedure; an undetected signal does not prove an exactly zero amount.<IUPAC Compendium of Chemical Terminology, fifth edition (Gold Book) by International Union of Pure and Applied Chemistry>[entry L03540, limit of detection; DOI 10.1351/goldbook.L03540]{ref} Moreover, a count of viral <RNA> is a molecular measurement, not a direct count of infectious <virions> or a complete description of disease state.
Back to article page
