If you’ve ever had a perfect blood pressure reading at your annual checkup, you probably left feeling confident. But what if that number was misleading? What if your blood pressure spikes outside the clinic and no one knows because no one’s checked? That’s exactly what happens in masked hypertension, a condition that may affect up to 1 in 5 adults and more than half of people already on blood pressure medications.

Masked hypertension is the reverse of “white coat” hypertension. Instead of elevated readings in the doctor’s office and normal readings at home, it’s the opposite: normal in the clinic but elevated at home, at work, or while you sleep. This silent pattern is easy to miss and profoundly dangerous. People with masked hypertension are at least twice as likely to suffer heart attacks, strokes, or kidney failure as those with truly normal blood pressure. And the scary part is that it behaves just like sustained high blood pressure in terms of long-term risk.

So who’s most at risk? You are, if you fall into any of the following categories: you have diabetes, chronic kidney disease, obesity, or you’re a man of African descent. Even if your office blood pressure is “normal,” if it’s close to the threshold (say, 125/78 mmHg), that’s a red flag. Masked hypertension is also surprisingly common in people already taking medication for high blood pressure, a phenomenon known as masked uncontrolled hypertension. In these cases, it means the treatment is failing quietly in the background.

The consequences are not theoretical. Elevated blood pressure that flies under the radar can silently thicken the walls of your heart (a condition called left ventricular hypertrophy), damage your kidneys (often seen as protein in the urine), and stiffen your arteries. These are early signs of organ damage, often present even before sustained hypertension is diagnosed.

The problem is that masked hypertension can’t be caught in the office. You need to look at blood pressure in the real world when you’re asleep, working, stressed, or relaxing at home. The gold standard for this is ambulatory blood pressure monitoring (ABPM): a 24-hour test that takes readings every 15 to 30 minutes during your day and night. Alternatively, self-measured blood pressure (SMBP) at home with a validated cuff can also catch this pattern, especially if done over multiple days. But a single clinic reading simply isn’t enough.

So what should you do? If your office blood pressure is borderline, or if you’re already being treated for hypertension but have any sign of organ damage, or if you just want to know your true risk, start with out-of-office monitoring. This isn’t just for those with obvious hypertension. Guidelines now recommend screening for masked hypertension in anyone at high risk of cardiovascular disease, including people with diabetes, kidney disease, and elevated cholesterol.

Treatment decisions should be based on these real-world numbers. If out-of-office readings confirm masked or masked uncontrolled hypertension, that’s your cue to act. Adjusting your medications or starting therapy can reduce your risk of heart attacks, strokes, and kidney failure. But treatment should always be guided by your actual blood pressure pattern, not just what shows up during your 10-minute visit at the clinic.

The bottom line: masked hypertension is common, dangerous, and easily missed, but not if you look for it. If you’re serious about preventing cardiovascular disease, it’s time to ask not just what your blood pressure is in the office, but what it’s doing when no one’s watching.