Evaluation of a patient with hypoventilation requires a combination of clinical history, physical examination, pulmonary function testing, and chest radiography to help determine the cause. Specialized testing such as measurement of respiratory muscle strength and assessment of ventilatory control may also be needed. Genetic testing may help make the diagnosis of some disorders such as the central congenital hypoventilation syndrome. In some patients the first laboratory clue that chronic hypoventilation is present is to note an unexplained elevation in the serum CO (2) (HCO (3)) on routine electrolyte testing. Nocturnal oximetry and polysomnography are usually required to determine if obstructive or central sleep apnea is present in addition to nocturnal hypoventilation. In addition, the severity of daytime hypoventilation or pulmonary function impairment often does not accurately predict the severity of nocturnal changes in arterial oxygen saturation and the degree of nocturnal hypoventilation. End-tidal PCO (2) and transcutaneous PCO (2) are sometimes utilized to directly estimate the degree of nocturnal hypoventilation during sleep studies. They have limitations but may be especially useful to detect trends in the PCO (2) during the night.