RNA quantification methods are broadly used in life science research and in clinical
diagnostics. Currently, real-time reverse transcription polymerase chain reaction
(RT-qPCR) is the most common analytical tool for RNA quantification. However, in cases
of rare transcripts or inhibiting contaminants in the sample, an extensive amplification
could bias the copy number estimation, leading to quantification errors and false
diagnosis. Single-molecule techniques may bypass amplification but commonly rely on
fluorescence detection and probe hybridization, which introduces noise and limits
multiplexing. Here, we introduce reverse transcription quantitative nanopore sensing
(RT-qNP), an RNA quantification method that involves synthesis and single-molecule
detection of gene-specific cDNAs without the need for purification or amplification.
RT-qNP allows us to accurately quantify the relative expression of metastasis-associated
genes MACC1 and S100A4 in nonmetastasizing and metastasizing human cell lines, even at
levels for which RT-qPCR quantification produces uncertain results. We further
demonstrate the versatility of the method by adapting it to quantify severe acute
respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA against a human reference gene. This
internal reference circumvents the need for producing a calibration curve for each
measurement, an imminent requirement in RT-qPCR experiments. In summary, we describe a
general method to process complicated biological samples with minimal losses, adequate
for direct nanopore sensing. Thus, harnessing the sensitivity of label-free
single-molecule counting, RT-qNP can potentially detect minute expression levels of RNA
biomarkers or viral infection in the early stages of disease and provide accurate
amplification-free quantification.