Wearable Health Devices: What Your Apple Watch and Fitbit Can (and Can't) Tell You

The Doctor on Your Wrist

Consumer health wearables have evolved from simple pedometers into sophisticated devices capable of measuring heart rate, detecting irregular heart rhythms, estimating blood oxygen levels, tracking sleep patterns, and even recording single-lead electrocardiograms. Millions of Americans now wear devices from Apple, Fitbit, Garmin, Samsung, and other manufacturers that generate a continuous stream of health data.

But how accurate is this data, and when does it actually help your healthcare? At Zimmer Medical Group, we welcome patients who bring wearable data to their appointments, and we want to help you understand what your device can and cannot tell you.

What Current Devices Can Measure

Heart Rate Monitoring

Most wearables use photoplethysmography (PPG), which shines green LED light into the skin and measures reflected light to detect blood volume changes with each heartbeat. This technology provides:

• Resting heart rate: Generally accurate within 3 to 5 beats per minute under resting conditions. Trends over time are more useful than any single reading.
• Exercise heart rate: Accuracy decreases during vigorous activity, especially exercises involving wrist movement. Chest strap monitors remain more accurate during intense workouts.
• Heart rate variability (HRV): The variation in time between heartbeats, used as an indicator of autonomic nervous system function, stress, and recovery. While the raw measurement is reasonably accurate, the health interpretations that apps build on top of HRV data are still evolving.

ECG and Atrial Fibrillation Detection

The Apple Watch, Samsung Galaxy Watch, and some Fitbit devices can record a single-lead electrocardiogram and screen for atrial fibrillation (AFib). The FDA has cleared (not approved) these features, meaning the agency reviewed them and found they perform reasonably well for their intended screening purpose.

Key points about wearable ECGs:

• They detect AFib with roughly 98 to 99 percent sensitivity in clinical studies, meaning they rarely miss true AFib
• They are designed for screening, not diagnosis. A positive result requires confirmation with a clinical 12-lead ECG
• They cannot detect heart attacks, most arrhythmias other than AFib, or structural heart problems
• They work best for people with suspected but undiagnosed AFib. They are not recommended as a screening tool for the general population without risk factors

Blood Oxygen (SpO2)

Wearable pulse oximeters estimate blood oxygen saturation using red and infrared light. While helpful for general trends, they are:

• Less accurate than medical-grade finger pulse oximeters, particularly at lower oxygen levels where accuracy matters most
• Affected by skin pigmentation, poor fit, cold temperatures, and motion
• Not reliable enough for clinical decision-making about respiratory conditions, sleep apnea diagnosis, or altitude sickness

If you have a lung condition, heart disease, or sleep apnea, use a medical-grade pulse oximeter recommended by your doctor rather than relying on your watch.

Sleep Tracking

Modern wearables estimate sleep stages (light, deep, REM) using a combination of heart rate, movement, and sometimes blood oxygen data. Research shows:

• Total sleep time estimates are reasonably accurate, typically within 15 to 30 minutes
• Sleep stage classification is significantly less accurate than polysomnography (the gold standard sleep study). Devices tend to overestimate deep sleep and underestimate wake time
• Sleep trends over weeks and months can be useful for identifying patterns and the effects of lifestyle changes on sleep quality

Step Counting and Activity Tracking

Step counting is one of the most mature and accurate wearable features, typically accurate within 5 to 10 percent. Calorie expenditure estimates, however, can be off by 20 to 40 percent or more, as they rely on algorithms that cannot account for individual metabolic variation, body composition, or exercise efficiency.

Fall Detection

Apple Watch and some other devices include fall detection that can automatically call emergency services. While imperfect, this feature has documented real-world instances of alerting help after falls, which can be particularly valuable for older adults living alone.

FDA Clearance vs. FDA Approval

Understanding this distinction matters. Most wearable health features receive FDA 510(k) clearance, which means the device is substantially equivalent to an existing legally marketed device. This is a lower regulatory bar than FDA approval, which requires clinical trials demonstrating safety and effectiveness. Clearance means the device is reasonable for its intended use, not that it is a clinical diagnostic tool.

The FDA consumer guidance on health wearables provides helpful context for understanding what these devices are designed to do.

When Wearable Data Is Clinically Useful

Your wearable data can genuinely help your healthcare in several scenarios:

• Capturing intermittent symptoms: If you experience occasional palpitations, dizziness, or irregular heartbeats, a wearable ECG recording during symptoms can provide your doctor with valuable diagnostic information
• Tracking trends: Resting heart rate trends, sleep patterns, and activity levels over weeks to months reveal meaningful health patterns
• Motivating behavior change: Patients who track their steps, activity, and sleep often make healthier choices. The data provides accountability and visible progress
• Monitoring chronic conditions: Patients with AFib, heart failure, or other conditions may benefit from continuous heart rate monitoring between office visits

When Wearable Data Causes Unnecessary Anxiety

Wearables can also create problems:

• False positives: Irregular rhythm notifications in healthy, young individuals often reflect artifact or benign variations, leading to unnecessary anxiety, emergency room visits, and expensive follow-up testing
• Over-monitoring: Checking your heart rate or oxygen level dozens of times per day can feed health anxiety. If you find yourself frequently checking your device with worry, discuss this with your doctor
• Misinterpreting normal variation: Heart rate variability, resting heart rate, and blood oxygen naturally fluctuate throughout the day. A single low or high reading is rarely meaningful
• Sleep anxiety: Ironically, some people develop insomnia from worrying about their sleep scores

How to Share Wearable Data with Your Doctor Effectively

If you want to bring wearable data to your appointment, follow these guidelines:

1. Focus on trends, not single readings. A weekly or monthly summary is more useful than a screenshot of one measurement.
2. Highlight specific concerns. "My resting heart rate has increased from 65 to 80 over the past month" is actionable. "Here are 500 screenshots" is not.
3. Use the device's built-in health summaries or export features to create concise reports.
4. Correlate data with symptoms. "I felt palpitations at 3 PM Tuesday, and here is the ECG I captured" is extremely valuable.
5. Ask your doctor which metrics are relevant for your specific health conditions. Not every measurement matters for every patient.

The Bottom Line

Wearable health devices are powerful tools for health awareness and motivation, but they are screening tools, not diagnostic instruments. They are most valuable when used to identify patterns, capture intermittent events, and motivate healthier behaviors, and least valuable when used to self-diagnose or replace medical evaluation.

The American Heart Association supports the use of wearable devices as supplementary health tools while emphasizing that clinical evaluation remains the standard for diagnosis and treatment decisions.

Have questions about what your wearable device is telling you? Contact Zimmer Medical Group to schedule an appointment. We can help you interpret your data, decide which metrics matter for your health, and integrate wearable insights into your care plan.