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Synchronous telemedicine (real-time interactive telehealth) is the delivery of clinical care or clinical trial visits through live, two-way audiovisual or audio-only telecommunications where the clinician and patient interact simultaneously. By contrast, asynchronous telehealth (store-and-forward telemedicine) captures clinical data—such as digital dermatology photographs, DICOM imaging studies, prerecorded videos, or structured patient questionnaires—and transmits them to a practitioner for evaluation at a later time without requiring both parties to be present concurrently.
What Is Synchronous and Asynchronous Telehealth?
Under U.S. Centers for Medicare & Medicaid Services (CMS) regulations at 42 CFR § 410.78, synchronous and asynchronous telehealth represent two distinct statutory definitions of telecommunications technology:
- Synchronous Telehealth (Interactive Telecommunications System, 42 CFR § 410.78(a)(3)): Multi-media communication equipment that includes, at a minimum, audio and video equipment permitting two-way, real-time interactive communication between the patient at the originating site and the physician or non-physician practitioner at the distant site. CMS also permits synchronous two-way audio-only interactions for specific mental health visits and qualifying home-based encounters when the distant-site practitioner has technical audiovisual capability but the patient is unable to use or does not consent to video.
- Asynchronous Telehealth (Store-and-Forward Technologies, 42 CFR § 410.78(a)(4)): The asynchronous transmission of medical information from an originating site to be reviewed at a later time by a physician or practitioner at a distant site, where the practitioner renders a medical evaluation or management service without a concurrent live interaction with the patient.
Synchronous vs. Asynchronous Telemedicine: Side-by-Side Comparison
Choosing between synchronous and asynchronous modalities in clinical care and decentralized clinical trials depends on physical examination requirements, diagnostic urgency, reimbursement rules, and network constraints:
| Dimension | Synchronous Telemedicine (Real-Time) | Asynchronous Telemedicine (Store-and-Forward) |
|---|---|---|
| Interaction Timing | Simultaneous, live two-way interaction requiring coordinated appointment scheduling | Time-decoupled; data capture and specialist interpretation occur at separate times |
| Clinical Use Cases | Primary care E/M visits, telepsychiatry, acute telestroke (NIHSS), genetic counseling, real-time telecardiology echocardiography guidance, and clinical trial informed consent | Teledermatology lesion review, teleradiology PACS reading, diabetic retinopathy teleretinal screening, async ECG interpretation, and eConsults |
| Core Transport Protocols | Low-latency WebRTC or SIP media streams encrypted with DTLS-SRTP or AES-256-GCM |
TLS-encrypted REST/FHIR APIs, DICOMweb (STOW-RS / WADO-RS), and store-and-forward message queues |
| Hardware & Bandwidth Needs | Continuous symmetric broadband (typically ≥1.5–15 Mbps) with telehealth equipment such as PTZ cameras and real-time digital stethoscopes | Tolerates intermittent connectivity; high-resolution files upload in the background before review |
| Medicare CPT / HCPCS Codes | Office/outpatient E/M codes (CPT 99202–99215), telehealth consultations, and behavioral health codes |
HCPCS G2010 (recorded video/image review), CPT 99421–99423 (online digital E/M), and CPT 99451–99452 (interprofessional eConsults) |
| Claim Modifiers & POS | Modifier 95 (live audio-video), Modifier 93 (live audio-only), or GT; Place of Service (POS) 02 or 10 |
Modifier GQ (federal demonstration store-and-forward) where applicable; standard non-telehealth POS for digital E/M |
| DEA Controlled Substance Prescribing | Qualifies as a bona fide telemedicine encounter under the Ryan Haight Act (21 U.S.C. § 829(e)) and DEA telehealth flexibilities |
Does not satisfy DEA Ryan Haight Act interactive evaluation requirements; static online questionnaires alone are prohibited for controlled substances |
Billing, Coding, and Regulatory Differences
Health systems and research billing offices must maintain strict coding separation between synchronous and asynchronous encounters under telemedicine compliance policies:
- Synchronous Medicare Billing Rules: Synchronous office/outpatient evaluation and management visits (
CPT 99202–99215) are billed with POS 10 when the patient is located in their home or POS 02 when the patient is at another facility/location. Appending Modifier 95 attests that the service was rendered via real-time interactive audio and video telecommunications, whereas Modifier 93 attests to an approved synchronous audio-only interaction. - Asynchronous Medicare Billing Rules: Under
42 CFR § 410.78(d), traditional Medicare statutory store-and-forward substitution for full telehealth services with Modifier GQ was historically restricted to federal telemedicine demonstration programs in Alaska and Hawaii. However, CMS reimburses nationwide asynchronous communication-technology-based services (CTBS) for established patients, including HCPCS G2010 (remote evaluation of recorded video or images submitted by an established patient, including interpretation and follow-up within 24 business hours) and online digital evaluation and management services (CPT 99421 for 5–10 minutes, CPT 99422 for 11–20 minutes, and CPT 99423 for ≥21 minutes of cumulative clinician time over 7 days). - State Medicaid Variations: Outside Medicare, more than 35 state Medicaid programs reimburse asynchronous store-and-forward services—most commonly in teledermatology, teleophthalmology, and dental tele-screening—as well as asynchronous provider-to-provider eConsults (
CPT 99451).
Hybrid Synchronous-Asynchronous Clinical Workflows
Modern virtual care programs and decentralized clinical trials rarely rely on a single modality in isolation. Instead, they combine asynchronous data collection with targeted synchronous encounters:
- Asynchronous Pre-Visit Intake and Triage: Patients complete structured electronic patient-reported outcome (ePRO) instruments, upload medication photos, or transmit home physiological readings before the visit.
- Synchronous Diagnostic or Therapeutic Encounter: The clinician conducts a live audiovisual visit to perform targeted history-taking, guided physical examination maneuvers, shared decision-making, or electronic informed consent (
eConsent) verification. - Asynchronous Specialist eConsult or Follow-Up: If subspecialty input is needed, the primary clinician packages the encounter summary and diagnostic images into an asynchronous interprofessional consultation (
CPT 99451–99452) or schedules asynchronous post-visit portal check-ins (CPT 99421–99423).
Frequently Asked Questions
What is synchronous and asynchronous telehealth?
Synchronous telehealth is real-time virtual care where the clinician and patient communicate live at the same time over two-way video or audio. Asynchronous telehealth (store-and-forward telemedicine) is time-separated virtual care where clinical images, diagnostic studies, or structured messages are recorded and transmitted to a clinician for review and medical decision-making at a later time.
What is the difference between synchronous and asynchronous telehealth?
The primary differences lie in timing, clinical examination capability, and billing rules. Synchronous telehealth requires simultaneous participation, supports interactive examination and real-time questioning, qualifies for standard E/M CPT codes (99202–99215) with Modifier 95 or 93, and satisfies DEA interactive telemedicine requirements. Asynchronous telehealth eliminates simultaneous scheduling, uses store-and-forward codes (HCPCS G2010, CPT 99421–99423, CPT 99451), and cannot be used alone to prescribe controlled substances under federal Ryan Haight Act rules.
