The threshold for low cortisol levels can vary depending on the individual and the laboratory conducting the test. Validated testing chemistries and lab equipment are specific and unique to each laboratory developed test (LDT) and testing brand, because of this reported lab results on the same patient can vary from one testing brand to the next. However, according to research by Aardal & Holm, 1995, in general, cortisol levels between 0.094–1.551 µg/dL are considered the normal range for morning salivary measurements.

Aardal, E., & Holm, A. C. (1995). Cortisol in saliva–reference ranges and relation to cortisol in serum. European journal of clinical chemistry and clinical biochemistry : journal of the Forum of European Clinical Chemistry Societies, 33(12), 927–932.
https://doi.org/10.1515/cclm.1995.33.12.927