The iHealth Dock BP3 monitors blood pressure. (Courtesy of iHealth Labs)

The modern health-device craze started with pedometers that simply tracked the number of steps you took in a day. Now there are hundreds of gadgets that you can slap on your wrist or tuck into your pocket.

They sync up to your smartphone and track everything from blood pressure and calories burned to how stressed you are and how well you slept last night.

But how effective are those gadgets in keeping you healthy? “That’s the big question,” says Steven Steinhubl, a cardiologist and director of the digital medicine program at Scripps Health in La Jolla, Calif.

He says published research shows that some devices on the market still have kinks to iron out. For example, one lets you take a photo of a skin lesion with your smartphone and tells you whether it’s cancerous. But it got the diagnosis wrong a third of the time.

The devices that experts say are most useful right now are those that help monitor common, chronic conditions. For example, a study at the University of Florida found that home blood pressure monitors and blood glucose meters gave doctors valuable information to help them treat people with hypertension and diabetes.

The Fitbit One activity tracker counts steps and logs calories. (Courtesy of Fitbit)

The devices also helped patients get more involved in their own care.

In a study published in the Annals of Thoracic Surgery, older patients recovering from heart surgery wore activity trackers, which are devices that count steps taken and calories burned.

The patients who took the most steps early on had shorter hospital stays. The researchers concluded that trackers might motivate patients to move more after surgery, which can speed recovery.

Here are three devices that Consumer Reports found accurate in its tests. All can be synced to your computer, smartphone or both, though sometimes that requires buying other software.

Blood pressure monitor: iHealth Dock BP3, $100

What it does: Allows you to track and store your blood pressure and heart-rate results on your iPhone or iPad and to share the information with your doctor. It also offers a blood pressure risk indicator and a detector for irregular heartbeats.

Best for: Older adults, whose blood pressure can vary; people whose blood pressure tends to spike when tested in a doctor’s office (a condition called white-coat hypertension); people with diabetes, for whom blood pressure monitoring is important.

The Accu-Chek Aviva Expert monitors blood glucose and includes a built-in bolus calculator. (Courtesy of Accu-Chek)

Blood glucose meter: Accu-Chek Aviva Expert, $20

What it does: Stores up to 500 readings of your blood glucose levels and calculates averages over time. Users can flag their results as pre- or post-meal for reference.

Best for: People with Type 1 or Type 2 diabetes who take insulin.

Activity tracker: Fitbit One, $100

What it does: Counts steps, logs calories, monitors sleep and tells you how close you are to meeting daily health goals. It also has a clock and an alarm.

Best for: People who want a motivational tool to help them become more active.

Copyright 2014. Consumers Union of United States Inc.

For further guidance, go to www.ConsumerReports.org/Health, where more detailed information, including CR’s ratings of prescription drugs, treatments, hospitals and healthy-living products, is available to subscribers.