Microfluidic sensing is of interest for biomedical applications, and for monitoring trace contaminants in the environment. However, a sensor offering physical robustness, simple integration into microfluidic systems, high sensitivity, and good analyte specificity is lacking. The authors present theory and experiments on a lasing sensor that can address many of these requirements, with sensitivity of over 1000 nm of wavelength shift per refractive-index unit in robust, untreated capillaries, and the potential for surface-based specificity. This work could lead to a fresh direction in research on capillary-based microfluidic sensors, and an alternative to whispering-gallery devices.