HST: The Elephant in Sleep Medicine

The Elephant
Blindfold three people and ask them to feel an elephant and to describe it. The first, feeling a leg, may say it appears to be like a tree, another describes the elephant’s trunk as a hose, and the third thinks the body may be a great wall. They are all right and they are all wrong. It is not until each has completed a full inspection that they can agree on the true nature of the beast.

And so it is with the home sleep test (HST), or perhaps more accurately, the “portable or limited channel, unattended sleep test.” The controversy surrounding HST is arguably more polarized than the current political agendas. Witness many articles, discussions at meetings, and hundreds of spirited comments posted on LinkedIn and other Internet threads. Opinions on the use of HST in sleep medicine range from “never use” to “use as a first approach.” The stakeholders include patients, physicians, dentists, other healthcare service providers, sleep testing and treatment facilities, equipment manufacturers, employers, third party payers, regulatory agencies, attorneys and society in general. The intent of this article is to incorporate the different views of the stakeholders about some of the fundamental issues related to HST, put the use of HST into a perspective that makes sense, and to propose an algorithm as an initial guideline to address the use of and results obtained from HST.

The Issue
The increasing awareness of the consequences of undiagnosed and untreated obstructive sleep apnea (OSA) and the frequency of the condition have received a lot of attention in the media. The public, employers, regulators, attorneys, and insurance companies are now more aware of healthcare costs and liability associated with OSA. These issues as well as accessibility are driving the sleep industry to perform more at-home sleep tests (HST).

HST has many advantages. It costs much less than facility-based polysomnography, is more acceptable to patients, easier to schedule, does not interfere with daily activities and is performed in the comfort of the patient’s home or other location reducing the “first night effect” which may influence the results of a facility-based study. The HST allows more rapid turnaround of results, provides more access for many patients in need of diagnosis and treatment, and could allow some patients to start on auto-adjusting CPAP (APAP) or an oral appliance without the need for PAP titration.

On the other hand, HST is a limited channel and unattended study. It’s imperative that patients are properly selected and that appropriate protocols based on the HST results are in place so that further diagnostic testing is recommended when indicated.

The Purpose
The purpose of this article is not to rehash details of the HST controversy or specifically to present new data. The HST wave has arrived and appears to be here to stay in spite of any arguments to the contrary. We have to accept that CMS and other payers now recognize HST as a diagnostic rather than a screening tool, so the industry must learn to work with this. It is more important to put HST in perspective, like any other diagnostic procedure in the medical tool box, and to help stakeholders fit HST into their tool boxes so that this elephant can be tamed, appreciated for what it is, and take its place alongside other diagnostic tests used appropriately for the benefit of the patient.

To this end, I am proposing an algorithm or decision tree for interpretation of a Level III limited channel HST and for recommendations based on the results of the HST. While the kernel of the algorithm is a decision tree, it is by necessity integrated within a larger, cohesive care plan including education, screening, testing, treatment, monitoring, and outcome evaluation. It also considers economic, operational and accessibility issues, among others. I hope this suggested approach will serve as a roadmap to be modified according to the needs of healthcare providers and their particular industries or patient populations, continue to evolve as the result of experience and outcome evaluations, and ultimately be incorporated into more formal recommendations by professional societies, payers and other interested parties. A section on APAP is included since some payers have already tied HST to APAP.

Equipment and Limitations and Alphabet Soup
Any limited channel screening device is subject to certain limitations. As EEG is not typically recorded, total sleep time is not measured, and while some units record motion or other indicators, there is some disagreement among sleep medicine professionals as to the degree to which these measurements are reliable estimates of sleep time. Since most economically feasible HSTs do not measure actual sleep time, the apnea-hypopnea index (AHI) is calculated using total recording time (TRT) rather than total sleep time (TST), so the AHI derived from the HST is calculated as apneas + hypopneas/TRT. This means that if the AHI from a HST is ≥ 5, the “true” AHI is underestimated by the HST since TRT is usually > TST and false negatives would be less likely. Therefore, the “true” AHI (sum of apneas and hypopneas/hour of sleep) and thus the severity of sleep apnea will be underestimated — and the lower the sleep efficiency (TST/TRT x 100), the greater the underestimate will be.

On the other hand, if an AHI derived from the HST in a “high risk” individual is < 5, it would more likely be a false negative (assuming that an AHI ≥ 5 is considered the threshold for sleep apnea). As with any single night study, including attended polysomnography (PSG), the severity of sleep apnea may also be underestimated due to lack of or limited supine and/or REM sleep.

Unfortunately, the inaccurate use of the abbreviations for apnea hypopnea index (AHI) and respiratory disturbance index (RDI), sometimes used interchangeably, has been pervasive in the industry. This common practice has led to some inconsistent definitions among manufacturers of HST (and PSG) devices, payers, scientific papers, and PAP machine download data reports as well. The implementation of AASM scoring rules should help standardize these definitions. To clarify the conventional terminology:

AHI = apneas + hypopneas per unit of sleep time in hours (technically only measured by “real” EEG)

RDI = apneas + hypopneas + RERAs (respiratory effort related arousals) per unit of sleep time in hours

Furthermore the RDI, sometimes reported with HST, is not really correct as arousals cannot be accurately detected without EEG. Also, flow limitation based on waveform shape has sometimes incorrectly been incorporated into RDI derived from HST, and some have incorporated snoring into RDI. In the case of PSG, snoring with EEG arousals is sometimes included as a respiratory event into RERAs.

A recent review article has addressed many technical aspects of OSA devices for out-of-center testing (OCC) and proposed a new classification system for measurement parameters. According to this article, the term respiratory event index (REI) is defined, in the context of OCC devices, as follows:

REI = apneas + hypopneas/total sleep or recording time in hours.[1]

Having said this, the term AHI (as currently reported with most HST) will be used here for convenience and in most instances will mean REI using TRT as the denominator. For the above reasons, the term RDI, although sometimes equivalent, will not be used here. Clearly there remains the need for standardized terminology.

The minimum required channels for Level III HST are airflow, oxygen saturation (SpO2), pulse rate, and respiratory effort. Optional channels may include snoring, body or head position, motion detection, e.g. actigraphy, light sensor, pulse plethysmography, peripheral arterial tonometry, pulse transit time and, in some cases, extra lead(s) for some combination of EEG, EOG, EMG, or EKG. An indicator of signal quality and the ability to perform HST during PAP would be a plus. For the most part, the minimum required channels are adequate to accomplish the purpose for which HST is intended–the diagnosis of (some still argue screening for) obstructive apnea. The optional parameters may be helpfulfor research, but their practical clinical contribution is diminished since most HSTs are single night studies and there is no control over a patient’s body position and sleep patterns on subsequent nights.

As addressed in more detail below, AHI from a HST (either ≥ or < 5) is not the only parameter on which the severity (and need for treatment) should be based. Oxygen saturation (SpO2) provides additional information in this regard, and clinically significant desaturations recorded during HST may be as or more important than AHI, since they indicate portions of the recording where respiratory events are more frequent in the face of a “normal” or mildly increased “average AHI” for the night. These events should be identified by review of the raw data. Patients that do not improve clinically on treatment(s) based on HST should have an attended PSG or PAP titration as appropriate.

Reporting and Data Analysis
While some manufacturers’ scored data reports are more flexible (customizable) than others, most include a good summary of the usual and customary parameters and indices.

Whether the HST is scored manually or automatically, the raw data should be reviewed by a qualified sleep physician who is ultimately responsible for the interpretation. It is interesting that some professional associations and payers have not made formal recommendations in this regard.

We have all seen situations where a HST (or PSG for that matter) shows an AHI < 5, but during a particular portion of the recording — perhaps 30 minutes or during supine and/or REM sleep — 20 respiratory events are scored, equivalent to an AHI of 40 for that segment. Thismay be associated with O2 desaturation of 80% or less. I would maintain that such a patient is in need of treatment as much or more than a patient with an AHI (TST) of 8 with no such “effectively increased” AHI period and with only mild desaturations. I don’t understand why certain third party payers do not cover PAP therapy in the former but do cover CPAP based on the “rule” of 4 hours for 70% of the nights (discussed below). CPAP adherence using this “rule” is not always ideal, as frequent respiratory events can occur during non-PAP-compliant sleep in the latter half of the night, particularly during REM sleep. However, this guideline persists because it is seen as better than nothing, although it could be detrimental to patient care.

Although many PSG systems report AHI for TST, NREM and REM, this is not always adequate to identify all situations in which AHI (TST) < 5 is associated with periods of more frequent respiratory events. In this regard, I suggest that all HST (or PSG) manufacturers include in their scored report a “maximum density index” or MDI which would display, for example, a 10 to 30 minute period from the recording during which the index is maximum for respiratory events (e.g. OA, HYP, CA or “AHI”), O2 desaturations, possibly snoring, heart rate, and arousals and leg movements (in the case of PSG). Some manufacturers do this already, in part. This would be particularly useful for HST with no true sleep staging. Other parameters, such as maximum, minimum, and mean (SD) O2 saturations and respiratory event durations, may aid in the HST interpretation and decision to recommend PSG.

The Algorithm
The algorithm is designed to analyze and act on data obtained by HST for the diagnosis of OSA and is generally based on a Level III, limited channel HST. It is not confirmed by controlled studies or consensus, since none exist at this time, nor has it been reviewed by committee. However, it has been modified based on input from several well qualified sleep specialists and from Internet thread discussions with others from various disciplines in the sleep industry. The suggested decision tree is not intended to be the last word but rather a roadmap for a reasonable approach. It is hoped that these suggestions, which may not always be in precise agreement with the policies of a particular payer, regulatory body or professional society, be taken in the light for which they are intended, modified as the provider sees fit, and incorporated into patient care plans more specifically tailored to the needs of a particular physician, patient population, industry, facility or organization. Of course, as with any diagnostic study, the patient’s treating physician should use the HST interpretation and recommendations in conjunction with the overall clinical evaluation and treatment of the patient. Hopefully with increased HST experience, outcome evaluations, clinical studies and advancements in technology, the approach suggested here will continue to evolve and be refined.

The Big Picture

Education –> Screening –> Testing –> Treatment –> Monitoring –> Outcome Evaluation –> Follow Up

The first part of the larger picture is the education of the medical community, the public and payers about the risks for and understanding of the consequences of undiagnosed and untreated obstructive sleep apnea as well as the many benefits of treatment. This subject has been covered extensively in many other venues.

Subjective validated questionnaires for assessing the risk of OSA such as the Epworth, STOP, STOP-BANG, Berlin, and/or other subjective screening tools should be used as they apply to specific patient groups or industries. Other considerations should include conventional history and physical examination as well as objective measures like BMI, neck circumference, cardio-pulmonary, upper airway and craino-facial evaluation. Specific inclusion criteria for medical necessity for HST may be important for reimbursement issues. There should also be screening criteria for exclusion of patients with certain co-morbidities or suspected co-existing sleep disorders in whom HST may not be appropriate. Unfortunately, some of the published exclusion criteria for COPD , CHF and heart disease are not well defined with respect to the severity of those conditions. Furthermore, some have excluded other suspected sleep disorders even in patients that are at high risk for OSA. No questionnaire or set of physical findings is perfect. It is clear that patients classified as “low risk” based on a particular set of signs and symptoms may in fact be at very high risk for OSA based on numerous other factors that are not included in commonly used criteria. The ultimate decision for HST should be based on the overall clinical evaluation by the patient’s treating physician.

HST- Sleep Apnea Decision Tree


< 5 Pre-Test Risk High »» Attended PSG or split night study if criteria met [1]

< 5 Pre-Test Risk Low »» Attended PSG or trial of conservative management as clinically indicated [1,2]

≥ 5 With Increased CA or CSR »» Attended PSG + Consider sleep specialist and/or cardio or neuro evaluation [3]

≥ 5 Using HST device that does not measure respiratory effort »» Attended PAP titration [4]

≥ 5 if none of above »» Begin Auto-PAP (APAP) trial (or Oral Appliance if not willing or able to tolerate APAP)

and if high AH or low SpO2 »» Attended PAP titration [5]

O2 Saturation (SpO2)

SpO2 during HST Normal »» Adherence program + Monitoring Downloads + Outcome evaluations [6,7]

SpO2 during HST Low »» Overnight Oximetry during APAP (or OA) + consider pulmonary function test (PFT) [8]

SpO2 corrected with APAP (or OA)

– Yes »» Adherence program + Monitoring Downloads + Outcome evaluations [7]

– No »» Adjust APAP (or OA) … Repeat overnight oximetry or attended PAP titration or per physician

1. High and low risk for OSA should be defined by the healthcare professional for specific patient groups or by industry requirements/standards
2. Weight loss, good sleep hygiene, education etc. and otherwise symptom related as determined by the treating physician
3. Evidence of Cheyne-Stokes Respiration (CSR) or increased central apneas (e.g. CA index ≥ 10 or ≥ 50% total respiratory events or per physician)
4. Caution should be used starting APAP if the HST device does not reflect central apneas and generally an attended PAP titration is recommended.
5. As defined by physician e.g. AHI ≥ 40 (example only) or SpO2 < 85% for ≥ 5 minutes or lowest SpO2 < 80% (example only)
6. Lowest SpO2 ≥ 90% (example only) or as defined by physician
7. See below for monitoring downloads
8. Any SpO2 during HST < 85-89% or mean <90% (examples only) or as defined by physician

Other Considerations
For excessive snoring that does not improve with APAP or OA or if the HST device does not record snoring, consider ENT evaluation. For irregular pulse or rate < 40 or > 100 bpm, consider EKG. If AHI (TRT) is < 5 but ≥ 5 during portions of the recording, consider APAP or attended PSG, depending on the severity. If there is no clinical improvement with APAP after a “reasonable” period of compliant APAP or oral appliance therapy, as determined by the physician, consider PSG, PAP titration, or consultation with sleep specialist as appropriate.

Perhaps (nominally) 5-20% of patients appropriately chosen for and properly tested by HST will eventually end up having PSG and/or PAP titrations for various reasons. These might include: high risk for OSA with “AHI” < 5, regardless of whether or not they actually have OSA or any respiratory events on the night of the HST; a false negative or an attempt to alter the outcome of the HST; severe OSA; significant central apneas are detected; or there is inadequate adherence or lack of clinical improvement on APAP. True, some will spend time and money on a HST and have to be retested, but if properly selected for HST, this number should be small and, on a societal level, many more people in need will be tested and treated a lower cost.

Monitoring Downloads and PAP Compliance
Data downloads from APAP (or CPAP) units should be reviewed at intervals after starting therapy. Generally I suggest 2-4 weeks, 3 months and up to every 3 months thereafter or more frequently if clinically indicated or mandated by industry or payer requirements. The data should be reviewed by the supplier of the PAP equipment and the physician to document not only adherence/compliance, but also mask leak and respiratory events. The physician should then adjust the PAP appropriately. If APAP download data indicate high pressures (e.g. ≥ 16 cm H2O) and/or there is no acceptable clinical improvement, an attended PAP titration study and/or consultation with a sleep specialist should be considered.

Current criteria for compliance/adherence with PAP therapy (i.e. 4 hours/night for 70% of the nights) are inadequate. First, if a patient uses PAP only in the first 4 hours of the night he/she will be “compliant,” but it is more likely that more frequent and more severe events will occur during REM, later in the night. Second, it has been reported that missing a single night of PAP can result in a measurable decrease in performance. Clearly there is need for a better system to access the effectiveness of PAP. Ultimately some economically feasible objective measure of outcome/performance not only would be more accurate but would eliminate the cost and inconvenience of current compliance monitoring practices.

Outcomes and Follow Up
In the end, improved quality of life and both subjective and objective clinical improvement are the ultimate goals.Subjective improvement can be accessed using questionnaires and objective improvement with performance testing such as the MWT, Psychomotor Vigilance Test (PVT) or driving simulators, although currently these are usually employed in research or are industry specific. If a patient is unwilling or unable to tolerate APAP, an oral appliance could be considered. One or more follow up HSTs may be necessary to assess the effectiveness of the appliance. If a patient is still symptomatic after a reasonable trial of adherent auto-CPAP and/or use of an oral appliance, as determined by the patient’s physician, an attended PSG or PAP titration and/or referral to a sleep specialist should be considered.

Final thoughts
The chain of custody (i.e., was the HST in fact performed on the intended patient?) is more of an issue in an industry where an employee’s job may be at stake. Unfortunately, answers on subjective questionnaires for assessing the risk of OSA in certain groups or industries may not always be accurate. Particularly in the commercial driving population, there may be an inclination to influence or alter the results of the HST. Much has been made of this issue but my view is that this is a procedural rather than a technical issue. Since only “high risk” individuals would undergo HST, a negative result (i.e. AHI < 5) should result in the recommendation for an attended PSG. Therefore the incentive to influence the HST result is minimized if a driver or other employee knows up front that a “negative” or “normal” HST, for any reason, will result in the recommendation for a PSG (and then probably a titration study) which will incur more cost and time.

There has been some discussion about the delay of diagnosis and treatment with HST, particularly if the equipment is delivered by mail. In very few situations is the diagnosis of OSA an emergency. The condition has likely existed for at least several years. Even in an acute pre-op situation, anesthesiologists are attuned to recognize patients at risk who will require heightened post-op observation. Although somewhat more important in the transportation industry, the real delay in the diagnosis and treatment of OSA lies with the millions of undiagnosed individuals due to some combination of accessibility, cost and education of healthcare providers and the public. It is precisely for these reasons that that HST, if appropriately used, can eventually help to reduce the time for diagnosis and treatment on a societal level.

The argument that HST should only be used in “full or real sleep medicine programs” defeats one of the main advantages of HST. The AASM acknowledged this with the February 2011 introduction of an accreditation program for out-of-center sleep testing (OSCT). Although a HST is more complex, in some ways it is no different than an EKG, spirometry or CBC. Clearly a primary care physician does not need to be board certified in cardiology, pulmonary medicine or hematology to utilize these studies in their overall management of a patient. If the treating physician is educated on identifying the risks of OSA, he could appropriately order the study as long as he ensures that qualified provider performs it and could use its results to manage his patient as long as they are interpreted by a physician trained in sleep medicine. As with any diagnostic test performed by a stand-alone lab or facility, the treating physician can decide if he/she is capable of managing the patient or prefers to refer the patient for more specialized evaluation and treatment. Many, if not most, DME companies and (non-sleep) physicians are not inclined to do the heavy lifting required to appropriately manage the OSA patient. Since there are simply not sufficient resources to serve the OSA population, stand alone testing programs and DME providers should develop more complete protocols and/or network with more advanced facilities in order to assure continuity of care and follow up whilst keeping the treating physician in the loop. In fact, several providers of sleep related products and services have been developing such “sleep communities” or networks which might also include OSA support organizations.


Batteries Not Included

Several closely related topics have not been covered in detail in this article, including:

• cost analysis
• billing
• coding and reimbursement
• medical necessity requirements
• federal and state regulatory or legal issues
• training, certification or licensure requirements for personnel
• data security, access, transmission, storage and retrieval
• operational issues
• marketing and business models
• adherence criteria
• auto vs. hand scoring of raw data
• comparison of specific HST devices
• disposable vs. reusable components
• accreditation issues
• technical and licensing policies of individual manufacturer’s scoring and reporting software.

These may be addressed in a future article.

Exaggerated Rumors
Mark Twain’s often-quoted comment, “The report of my death was an exaggeration,” may apply to some facility-based sleep testing programs that are concerned about the potential negative business impact of HST. Although a real and significant concern, we (hopefully) understand that HST as a diagnostic test for OSA has its limitations and, in a relatively small but significant number of situations, HST will not be sufficient to adequately identify and recommend treatment for certain sleep disorders. We also are aware that false negatives, coexisting sleep disorders and co-morbidities, if not identified, may lead to insufficient therapy. It is precisely for these reasons and because many more patients will be undergoing HST that more high-quality advanced sleep testing facilities will be required. However, to achieve this goal, providers of HST must follow well defined, reasonable protocols, both to identify medical necessity for those patients likely to benefit from HST and to recommend attended polysomnography and/or PAP titration and/or consultation with a sleep or other specialist if appropriate. We all know patients who initially may not be inclined to accept in-lab testing but will agree to undergo PSG or titration studies if an HST suggests OSA or another sleep disorder. As stated above I estimate that, nominally, 5 -20% of patients undergoing HST will eventually be referred for attended PSG or PAP titration. These situations should include a “negative” HST in patients with a high pre-test likelihood of OSA, an HST consistent with sleep apnea using HST equipment that does not measure central apneas, or a patient diagnosed with OSA by HST that does not improve with PAP or other therapy.

Be careful what you wish for
Whenever the sleep medicine industry has accepted — or been required by regulation to accept — diagnostic and therapeutic standards for specific groups of patients/employees, we have been obligated to implement and live by those recommendations for better or for worse. While the goals are to improve health and quality of life and to reduce accidents and liability all at lower cost, such “standards” must be well thought out as they can have considerable implications for patients and other stakeholders in the industry and society. By the same token the stakeholders should stand ready and be willing to readjust their positions based on new data, technology, patient outcome studies and economic changes.


Edward D. Michaelson, MD, FACP, FCCP, FAASM, is board certified in pulmonary medicine, internal medicine and sleep medicine. He has participated on local, state and national committees concerned with pulmonary medicine and has served as a consultant to the National Institute of Health, United States Air Force and the Divers Alert Network. He is the author of over fifty scientific publications and serves as medical director for sleep testing facilities.


1. Collop, NA et. al. Journal of Clinical Sleep Medicine, Vol 7, No. 5, 2011

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