The commonly used classification of sleep testing was proposed by Ferber et al. (
16) in a 1994 American Sleep Disorders Association review (
Table 44-1). This classification used the terminology Level I, II, III, and IV. This terminology was later modified to Type 1, 2, 3, and 4 (or Type I, II, III, and IV) (
17). In 2008, the CMS issued a decision to allow HST to qualify a patient for CPAP (
10,
11). CMS defined HST types slightly differently and created G-codes (G0398, G0399, and G0400) to describe HST services (
Table 44-2) (
11,
12). The G-codes are found in the Healthcare Common Procedure Coding System Level II codebook and are maintained and valued by the CMS. G-codes are procedure codes developed by the CMS to identify products, supplies, and services that do not have an assigned Common Procedural Terminology (CPT) code for
which there is a programmatic operating need to separately identify them on a national level. Reimbursement is determined regionally by the specific local coverage determination (LCD). Later HSATs were assigned CPT codes: 95800, 95801, and 95806 (
Table 44-3). The method of determination of sleep time (95800) is not specified. CPT codes are copyrighted and maintained by the American Medical Association. Although CPT codes exist for HSAT studies, many insurance providers require the use of G-codes for billing. The AASM uses the HSAT CPT codes in their recommended standards for such testing (
7,
13). The CPT classification of HSAT differs somewhat from the G-code classification. However, 95806 describes most Type 3 devices. Sleep testing using peripheral arterial tonometry (PAT), actigraphy, and oximetry was also approved for HST by the CMS, but a G-code was not assigned to this type of testing (
11). This type of testing would fall under the CPT classifications 95801 or 95802. An estimate of total sleep time (TST) is determined by PAT devices on the basis of actigraphy, heart rate, and characteristics of the sympathetic tone (PAT signal) in different sleep stages (
18). However, unless EEG and electrooculogram (EOG) are recorded, the AASM standards (
10) require that monitoring time (MT) rather than TST be reported. In the original classification, Type 4 testing required one or two bioparameter recording (
Table 44-1). However, in 2009 when the CMS issued a memo approving reimbursement for HST (
Table 44-2), Type IV devices were mandated to record three or more channels, one of which was airflow (
10,
11,
12). Most Type 4 devices monitor oximetry (oxygen saturation, SaO
2) as one of the channels being recorded.
A Type 3 study, also called “cardiorespiratory testing,” consists of at least two channels of respiratory monitoring, oximetry, and ECG or heart rate. The G0399 classification requires a minimum of four channels and must record ventilation, ECG/heart rate, and SaO
2. Typically, airflow and respiratory effort (e.g., chest and abdominal respiratory inductance plethysmography [RIP]) are used for respiratory monitoring. The CPT 95806 classification specifies monitoring of airflow and respiratory effort. Some of the simpler HSAT devices use only one respiratory effort belt. In most Type 3 devices, the heart rate is derived from the oximetry data. An example of a Type 3 study from a comprehensive HSAT device is shown in
Figure 44-1. When reading CMS documents, it is important to note that
the respiratory disturbance index (RDI) is the number of apneas and hypopneas per hour of MT. This RDI definition differs from the one used in the AASM scoring manual that specifies the RDI to equal the number of apneas, hypopneas, and respiratory effort-related arousals
per hour of TST (
10). The AASM also recommends that the term “respiratory event index” (REI) be used to describe the number of apneas and hypopneas per hour of MT in HSAT reports (
9,
13).