Background: The asthma burden is growing worldwide, and this is predisposed by environmental and occupational exposures as well as individual risk factors. This study was aimed at a comparison of diagnostic accuracy of spirometry and peak expiratory flow rate (PEFR) in asthma screening of adult patients with lung function abnormalities that present at the level of primary care. Methods: This study was conducted in Shymkent city, South Kazakhstan, the third most populous city of the country with developed industries and high rates of pulmonary diseases. Four hundred and ninety-five adult patients with lung function abnormalities were enrolled in the study and underwent two screening tests (spirometry and PEFR). The diagnosis of asthma was verified by a qualified pulmonologist after performance of screening tests and was based on symptoms, medical history, and laboratory and lung function tests. Results: The sensitivity of spirometry was 0.97 and that of PEFR was 0.95 (p = 0.721), whereas the specificity of spirometry was 0.37 and that of PEFR was 0.28 (p = 0.227). Both tests yielded the same results for the positive predictive value (0.98). The negative predictive value was significantly higher for spirometry versus PEFR (0.23 versus 0.08; p = 0.006). The positive and negative likelihood ratios of the two tests also differed significantly (p = 0.001 and p = 0.006, respectively), whereas the overall accuracy was comparable between the two tests (0.96 for spirometry and 0.94 for PEFR; p = 0.748). Conclusion: Ambulatory PEFR monitoring is non-inferior to the monitoring of the forced expiratory volume in 1 second and could be used for screening purposes on equal grounds with spirometry.