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GUIDE · 6 MIN READ
How to read CPAP data: six numbers, in order
Open a night of CPAP data and you can be shown forty things. Six matter for almost every decision you or your clinician will make, and they are best read in a fixed order, because each tells you whether the next is trustworthy. This guide to how to read CPAP data goes through those six: usage hours, AHI, leak, pressure, event composition and oxygen. A low AHI on a two-hour night means little; a high AHI on a heavy-leak night may be a mask problem, not an airway problem. So you read usage before AHI, and leak before you trust AHI.
Step 1: Usage hours. Did the machine have a chance?
What it is: how long you wore the mask with the machine running.
What to look for: 4 hours is the US compliance threshold (4 or more hours on 70 percent of nights over a 30-day window within the first 90 days). More is better. Most people sleep 6 to 8 hours; usage should be most of that.
Why it comes first: every other number is averaged over this time. A 90-minute night can show a wonderful AHI and tell you nothing. Before you read anything else, check that there was enough night to read.
In Apnealytics: usage hours sit at the top of the nightly card, and the Goals and streaks panel tracks the 4-hour line. A "Heads up" alert fires when usage slips.
Step 2: AHI. How many events per hour?
What it is: the Apnea–Hypopnea Index, the number of apneas and hypopneas per hour of use, as estimated by the machine from its flow sensor. It is an estimate, not a sleep study.
What to look for: under 5 is the conventional target on therapy. Apnealytics bands it: Great, Mild, Moderate, High. A single bad night is noise; three in a row is a pattern.
What not to do: react to one number. Look at the 30-night rolling trend before deciding anything changed.
In Apnealytics: last-night AHI with its band is the centre of the dashboard. The 30-night rolling AHI trend sits under it, and "What changed?" marks any point where the trend shifted or a setting changed.
Step 3: Leak. Can you trust the AHI?
What it is: air escaping the mask beyond the intentional vent flow. Reported as a rate in L/min. The useful summary is the 95th percentile, leak P95: the level that leak stayed under for 95 percent of the night.
What to look for: ResMed's large-leak threshold is 24 L/min. A P95 under that means the mask held nearly all night. Above it, the mask leaked heavily for at least 5 percent of the night, and during those stretches event detection and pressure delivery are both less reliable.
Why it comes before pressure and events: if leak was high, a high AHI may be the leak, not your airway. Fix the seal first.
In Apnealytics: leak P95 is on the nightly card. Mask Analytics shows seal, AHI and usage per mask.
Step 4: Pressure. What was the machine doing?
What it is: on an auto-adjusting machine (AutoSet, VAuto), the pressure moves between the minimum and maximum your clinician set. Pressure P95 is the level it stayed under for 95 percent of the night. The median tells you where it spent most of its time.
What to look for: compare P95 with your set maximum. If P95 sits at the maximum night after night, the machine may want to go higher than it is allowed; that is a conversation for your clinician. If P95 sits well below the maximum with a fine AHI, the range is comfortable. Also note EPR (expiratory pressure relief), which drops pressure as you breathe out.
In Apnealytics: pressure P95 is shown against your set range, with EPR beside it. "What changed?" flags any night the settings were altered.
Step 5: Event composition. What kind of events?
What it is: the AHI broken into obstructive apneas, central apneas, hypopneas and unclassified events.
What to look for: on CPAP, residual events are usually hypopneas and a few obstructives. A rising share of central apneas is worth noting: pressure does not treat centrals, and your clinician will want to know. The machine's labelling is an estimate from flow, so treat the split as a signal, not a verdict.
In Apnealytics: the event composition bar sits beside the AHI, and the per-night event strips on Pro show where each type clustered. The trigger journal lets you note alcohol, a cold or a new pillow and see whether AHI moved with it.
Step 6: Oxygen. Did any of it reach your blood?
What it is: SpO₂ from an oximeter worn overnight, overlaid on the therapy timeline. The summary numbers are ODI₃ and ODI₄ (desaturations of 3 or 4 points per hour) and T90 (minutes below 90 percent).
What to look for: low ODI and a T90 near zero mean the remaining events are not costing you oxygen. A high T90 with a low AHI is a mismatch worth raising with your clinician.
Why it comes last: it is the outcome. The first five describe what the machine did; this one describes what your body did.
In Apnealytics: oximetry overlays are Pro, from the Wellue O2Ring, Contec CMS50 family, ResMed SAD.edf or any EDF/CSV oximeter. Apple Health / Health Connect SpO₂ averages (free) appear in the Vitals tiles.
Putting it together: a worked night
Usage 7.2 h. AHI 2.1, Great. Leak P95 11 L/min. Pressure P95 10.4 against a 6–14 range, EPR 2. Events: 9 hypopneas, 4 obstructive, 2 central. ODI₃ 1.8, T90 0 min. Reading in order: enough night to judge, AHI low, leak low so the AHI is trustworthy, pressure in range, events unremarkable, oxygen held. Nothing to act on.
Now the same night with leak P95 at 38 L/min and AHI 7.4. Enough night, AHI Moderate, but leak was heavy, so the AHI may be a seal problem. Check Mask Analytics before worrying about the airway.
What to do with a pattern
One odd night: note it in the trigger journal and move on.
Three nights the same way: check "What changed?" and leak, then wait a week.
A trend over 30 nights: bring the chart to your clinician. The doctor-ready PDF on Pro puts the six numbers on one page.
Questions people ask
What is a good AHI on CPAP?
Under 5 events per hour is the conventional target on therapy, and many people run between 1 and 3. Read it alongside usage and leak: a low AHI on a short or leaky night is less meaningful. A single night above 5 is noise; a 30-night trend above 5 is worth a conversation with your clinician.
What is a normal leak rate on CPAP?
ResMed treats 24 L/min as the large-leak threshold. A leak P95 under that means the seal held for at least 95 percent of the night. Occasional spikes are normal, especially when you turn over. A P95 consistently above the threshold suggests the mask size, type or strap tension needs attention.
What does pressure P95 mean?
It is the pressure your auto-adjusting machine stayed at or below for 95 percent of the night. If it sits at your set maximum most nights, the machine may be capped. If it sits well below the maximum with a good AHI, the range is working. Changes to the range are a clinician decision.
Why is my AHI high even though I used the machine all night?
Check leak first. Heavy leak makes event detection and pressure delivery less reliable, so a high AHI on a high-leak night may be a seal problem. If leak is fine, look at event composition: a rise in central events is not treated by pressure and should be discussed with your clinician.
Do I need an oximeter to read my CPAP data?
No. The first five numbers come from the machine's SD card. An oximeter adds the outcome: whether residual events are costing you oxygen. It is most useful when AHI looks fine but you still feel unrested, or when central events are rising. Apnealytics overlays oximetry on Pro.
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