Your CPAP machine tracks several metrics every night, and learning to read them gives you a clear picture of how well your therapy is working. The most important number is your AHI (Apnea-Hypopnea Index), but leak rate, pressure data, and event breakdowns all tell part of the story. Here’s what each metric means and what to look for.
AHI: The Number That Matters Most
Your AHI tells you how many times per hour your breathing was partially or fully interrupted during sleep. It’s the single best indicator of whether your CPAP is doing its job. The American Academy of Sleep Medicine classifies untreated sleep apnea severity as mild (AHI of 5 to 14), moderate (15 to 30), and severe (above 30). On therapy, your goal is to get that number as low as possible, ideally below 5. Most people using CPAP well land somewhere between 1 and 4.
An AHI under 5 means your airway stayed open for the vast majority of the night. If your AHI creeps above 5 consistently, something needs attention: your mask fit, your pressure settings, or your sleep position. A single bad night doesn’t mean much. Look at your AHI trend over a week or more to get a reliable picture.
Obstructive vs. Central Events
Your machine doesn’t just count breathing disruptions. It categorizes them. Understanding the difference between the two main types helps you and your provider figure out the right response.
Obstructive events happen when your airway physically collapses or narrows, blocking airflow even though your body is still trying to breathe. Your chest and abdomen move, but air can’t get through. These are the events CPAP is specifically designed to prevent by splinting your airway open with pressurized air. If you’re seeing a lot of obstructive events, higher pressure or a better mask seal is usually the fix.
Central events are different. Your airway is open, but your brain temporarily stops sending the signal to breathe. There’s no chest or abdominal movement at all. A small number of central events is normal. If central events make up a large portion of your AHI, that’s worth discussing with your sleep specialist, because increasing CPAP pressure won’t help and can sometimes make central events worse.
Most CPAP machines label obstructive events as “OA” (obstructive apnea) and central events as “CA” or “Clear Airway.” Hypopneas, labeled “H,” are partial reductions in airflow. Your total AHI is the sum of all these events divided by hours of sleep.
Leak Rate: When Your Seal Breaks Down
Leak rate measures how much air escapes around your mask, reported in liters per minute (L/min). Every mask has some intentional leak built into its exhale port, and your machine accounts for that. The number you see is your unintentional leak: air escaping from a poor seal.
For ResMed devices, the threshold for acceptable unintentional leak is 24 L/min. Staying well below that is better. Even small leaks can cause problems you’ll feel, like air blowing into your eyes, dry mouth, or noise that wakes you or your partner. Users of ResMed’s myAir app who actively monitored their data averaged leak rates around 17 L/min, compared to roughly 19 L/min for less engaged users. That gap might sound small, but over a full night it can mean the difference between restful sleep and waking up with irritated eyes.
If your leak rate spikes during certain hours, you’re probably shifting position in your sleep and breaking the mask seal. A different pillow, mask style, or chin strap can help. Consistently high leak rates mean the mask doesn’t fit your face well, or cushions may need replacing.
Pressure Data and the 95th Percentile
If you use an auto-adjusting CPAP (APAP), your machine constantly raises and lowers pressure throughout the night based on what your airway needs. Your data report typically shows minimum pressure, maximum pressure, median pressure, and the 95th percentile pressure.
The 95th percentile is the pressure level your machine stayed at or below for 95% of the night. Sleep specialists treat this as the “effective” pressure, the level that keeps your airway open during your most vulnerable moments. It’s more useful than the maximum pressure, which might reflect a brief spike caused by a positional change or a swallow. If your 95th percentile pressure is consistently near the top of your prescribed range, your provider may need to adjust your settings. If it’s low and your AHI is also low, your therapy is working efficiently.
A difference of about 1.5 cm H₂O or more between your actual effective pressure and what your settings allow can affect how well your apnea is controlled and how comfortable you feel. If you notice your 95th percentile pressure climbing over weeks, it could signal weight gain, increased congestion, or changes in sleep position that are making your airway harder to keep open.
Flow Limitation: The Metric Most People Miss
AHI only counts events where your airflow drops significantly or stops entirely. But your airway can narrow enough to restrict airflow without triggering a scored apnea or hypopnea. This is called flow limitation, and many CPAP machines and third-party software like OSCAR can display it.
Flow limitation shows up as a flattening of the airflow waveform. In a normal breath, the flow curve looks rounded. When your airway is partially narrowed, the top of the curve flattens into a plateau. Research has shown that airway resistance during flow-limited breaths can increase by nearly 400% compared to normal breaths, even when AHI looks fine. This is why some people have a low AHI but still feel tired: their airway is working harder all night without technically crossing the threshold for a scored event.
If your CPAP software shows frequent flow limitation, a small increase in pressure or switching from a fixed pressure to auto-adjusting mode can help. This is especially relevant for people diagnosed with upper airway resistance syndrome, where the AHI is low but symptoms like daytime sleepiness and unrefreshing sleep persist.
Usage Hours and Insurance Compliance
Your CPAP tracks exactly how many hours you use it each night and how many nights per month you use it. This matters for two reasons: your health and your insurance coverage.
The widely used compliance standard requires at least 4 hours of use per night on at least 70% of nights over a 30-day period. Medicare and most private insurers use this benchmark to decide whether to continue covering your equipment. After the first 90 days of therapy, your provider needs to document that you’re meeting this usage threshold and that your symptoms are improving. Fall short, and you risk losing coverage for your machine, supplies, or both.
Tracking your own usage data helps you catch problems early. If you’re pulling the mask off at 2 a.m. every night, your total hours will reflect that, and you can troubleshoot the cause (pressure discomfort, dry mouth, mask irritation) before it becomes a compliance issue.
Where to Find Your Data
Most modern CPAP machines offer data in three places, each with different levels of detail.
- The machine’s built-in screen shows basic nightly summaries: AHI, usage hours, and leak rate. Navigate using the knob or buttons on the device. This is the quickest way to check last night’s numbers.
- Manufacturer apps like ResMed’s myAir or the Philips DreamMapper app pull data wirelessly from your machine and display it on your phone. These apps simplify the data into scores and tips. MyAir users average about an extra hour of nightly use compared to people who don’t track their data, and roughly 87% of users who pair the app with clinical monitoring hit their compliance targets.
- Third-party software like OSCAR (Open Source CPAP Analysis Reporter) reads the detailed data from your machine’s SD card. This is where you can see breath-by-breath flow curves, event timestamps, pressure graphs, and flow limitation patterns. It’s more complex, but it gives you the same granular data your sleep clinic sees.
The manufacturer apps are good for daily monitoring and motivation. If you want to understand why your AHI spiked on a particular night, or you’re trying to fine-tune your therapy, OSCAR gives you the depth to investigate. Download it free, insert your CPAP’s SD card into your computer, and import the data.
Putting It All Together
Reading CPAP data gets easier once you know what normal looks like for you. Check your numbers regularly for the first few months until you establish a baseline. After that, a weekly glance is enough unless something changes. The pattern to watch for: AHI under 5, leak rate well below 24 L/min, 95th percentile pressure stable, and usage above 4 hours. When one metric drifts, the others often follow. A rising leak rate can push AHI up because the machine can’t maintain effective pressure. A climbing 95th percentile might mean your airway needs more support than your current settings provide.
The goal isn’t to obsess over every data point. It’s to notice trends early enough to fix small problems before they become big ones. A mask cushion that’s worn out, a pressure setting that needs adjusting, or a sleeping position that’s working against you will all show up in your data before you feel the full impact during the day.