Physician input regarding LTOT use is frequently limited to giving authorization for liters per minute at rest (often an arbitrary flow of 2) and daily hours of use. Importantly, liter flow is merely a means to the clinical goal of appropriate oxygen-saturation levels. Physicians often confuse the Medicare Certification of Medical Necessity (CMN), or form DME 484.3, with the separate prescription required by state pharmacy boards for the administration of the drug, oxygen. CMN requires straightforward information such as primary and secondary diagnostic codes supporting use, the length of need from 1 to 99 months (lifelong), and Pao2- or oxygen-saturation test results. Medical necessity qualifying parameters were adapted from the Nocturnal Oxygen Therapy Trial9 study with regards to the degree of hypoxemia (Pao2 ≤ 55 mm Hg, arterial oxygen saturation [Sao2] or oxygen saturation by pulse oximetry [Spo2] ≤ 88%). Similarly, additional clinical and laboratory criteria9 were adapted to allow certification in the presence of sequelae from less severe hypoxemia (Pao2 = 56-59 mm Hg, Sao2 or Spo2 = 89%). A physician or an independent testing facility must perform these tests. Measurements can be made either during a chronic stable state as an outpatient or within 2 days prior to discharge from an inpatient facility to home. CMS only requires recertification at 12 months and when there is a substantial change in need (ie, continuous to exercise only). However, it is clinically sound to reassess oxygen need during a medically stable period following hospital discharge in patients who do not have preexisting hypoxemia. Either discontinuation or a change in the oxygen prescription may be necessary. The patient must be mobile within the home to meet the criterion for portable oxygen. The CMN statement is limited to “within the home” because, by statute, Medicare DME is intended only for home use. The clinical need for additional oxygen titration during exertion is not addressed by CMN with regard to patients requiring continuous oxygen therapy.