Editing Hypoxia (medicine) (section)

===Tests===
An [[arterial blood gas test]] (ABG) may be done, which usually includes measurements of oxygen content, hemoglobin, oxygen saturation (how much of the hemoglobin is carrying oxygen), arterial partial pressure of oxygen (P<sub>a<sub>O<sub>2</sub></sub></sub>), partial pressure of carbon dioxide (P<sub>a<sub>CO<sub>2</sub></sub></sub>), blood pH level, and bicarbonate (HCO<sub>3</sub>)<ref name="Cleveland ABG" />
*An arterial oxygen tension (P<sub>a<sub>O<sub>2</sub></sub></sub>) less than 80&nbsp;mmHg is considered abnormal, but must be considered in context of the clinical situation.<ref name="Bhutta et al 2022" />
*In addition to diagnosis of hypoxemia, the ABG may provide additional information, such as P<sub>CO<sub>2</sub></sub>, which can help identify the etiology. The arterial partial pressure of carbon dioxide is an indirect measure of exchange of carbon dioxide with the air in the lungs, and is related to minute ventilation. P<sub>CO<sub>2</sub></sub> is raised in hypoventilation.<ref name="Bhutta et al 2022" />
*The normal range of P<sub>a<sub>O<sub>2</sub></sub></sub>:F<sub>i<sub>O<sub>2</sub></sub></sub> ratio is 300 to 500 mmHg, if this ratio is lower than 300 it may indicate a deficit in gas exchange, which is particularly relevant for identifying [[acute respiratory distress syndrome]] (ARDS). A ratio of less than 200 indicates severe hypoxemia.<ref name="Bhutta et al 2022" />
*The [[alveolar–arterial gradient]] (A-a<sub>O<sub>2</sub></sub>,<ref name=loganabbrev /> or A–a gradient), is the difference between the [[Pulmonary alveolus|alveolar]] (A) [[concentration]] of [[oxygen]] and the [[artery|arterial]] (a) concentration of oxygen. It is a useful parameter for narrowing the differential diagnosis of [[hypoxemia]].<ref name="NBK545153" /> The A–a gradient helps to assess the integrity of the alveolar capillary unit. For example, at high altitude, the arterial oxygen P<sub>a<sub>O<sub>2</sub></sub></sub> is low, but only because the alveolar oxygen P<sub>A<sub>O<sub>2</sub></sub></sub> is also low. However, in states of [[ventilation perfusion mismatch]], such as [[pulmonary embolism]] or [[right-to-left shunt]], oxygen is not effectively transferred from the [[Pulmonary alveolus|alveoli]] to the blood which results in an elevated A-a gradient. P<sub>a<sub>O<sub>2</sub></sub></sub> can be obtained from the arterial blood gas analysis and P<sub>A<sub>O<sub>2</sub></sub></sub> is calculated using the [[alveolar gas equation]].<ref name="Bhutta et al 2022" />
*An abnormally low [[hematocrit]] (volume percentage of red blood cells) may indicate anemia.

[[X-ray]]s or [[CT scan]]s of the chest and airways can reveal abnormalities that may affect ventilation or perfusion.<ref name="MNT diagnosis" />

A [[ventilation/perfusion scan]],<ref name="Cleveland diagnosis" /> also called a V/Q lung scan, is a type of [[medical imaging]] using [[scintigraphy]] and [[Nuclear medicine|medical isotopes]] to evaluate the circulation of air and blood within a patient's [[lungs]],<ref name="UMMC" /><ref name="seminars"/> in order to determine the ventilation/perfusion ratio. The ventilation part of the test looks at the ability of air to reach all parts of the lungs, while the perfusion part evaluates how well blood circulates within the lungs.

[[Pulmonary function testing]]<ref name="MNT diagnosis" /> may include:
* Tests that measure oxygen levels during the night<ref name="MNT diagnosis" />
* The [[Pulmonary function testing#Oxygen desaturation during exercise|six-minute walk test]], which measures how far a person can walk on a flat surface in six minutes to test exercise capacity by measuring oxygen levels in response to exercise.<ref name="MNT diagnosis" />
* Diagnostic measurements that may be relevant include:<ref name="Temple" /> [[Lung volumes]], including lung capacity, [[airway resistance]], [[Muscles of respiration|respiratory muscle]] strength, [[diffusing capacity]] 
* Other pulmonary function tests which may be relevant include:<ref name="Temple" /> [[Spirometry]], [[body plethysmography]], forced oscillation technique for calculating the volume, pressure, and air flow in the lungs, [[Pulmonary function testing#Bronchodilator responsiveness|bronchodilator responsiveness]], [[Pulmonary function testing#Diffusing capacity|carbon monoxide diffusion test]] (DLCO), [[Oxygen therapy#Chronic conditions|oxygen titration studies]], [[cardiopulmonary stress test]], [[bronchoscopy]], and [[thoracentesis]]