A robust biomedical sensor for ultrasensitive detection of biomarkers in breath based on a tunable external cavity laser (ECL) and an off-axis cavity-enhanced absorption spectroscopy (OA-CEAS) using an amplitude stabilizer is developed. A single-mode, narrow-linewidth, tunable ECL is demonstrated. A broadly coarse wavelength tuning range of for the spectral range between 6890 and is achieved by rotating the diffraction grating forming a Littrow-type external-cavity configuration. A mode-hop-free tuning range of is obtained. The linewidths below 140 kHz are recorded. The ECL is combined with an OA-CEAS to perform laser chemical sensing. Our system is able to detect any molecule in breath at concentrations to the ppbv range that have absorption lines in the spectral range between 1450 and 1620 nm. Ammonia is selected as target molecule to evaluate the performance of the sensor. Using the absorption line of ammonia at , a minimum detectable absorption coefficient of approximately is demonstrated for 256 averages. This is achieved for a 1.4-km absorption path length and a 2-s data-acquisition time. These results yield a detection sensitivity of approximately . Ammonia in exhaled breath is analyzed and found in a concentration of 870 ppb for our example.