WASHINGTON – Patients checking into hospitals and clients meeting with their lawyers often are advised to take a simple step toward planning for the future: fill out a living will.

But the reality is, too often, they don’t really work.

The concept is straightforward: outline your wishes while you’re healthy to guide doctors and family members who might have to make life-or-death decisions for you later on. Would you want to linger in a vegetative state if it were unlikely you’d ever wake up? Or would you rather be allowed to die?

But end-of-life medical questions often involve much more complicated medical and moral questions. Many living wills are so vague as to be useless.

“We’ve tried to take very complicated, difficult, painful, wrenching decisions and make them a simple yes or no. These are never easy decisions and they’re never black-and-white,” said Sean Morrison, a geriatrician at Mount Sinai Medical School in New York.

A will might direct that no “extraordinary measures” be used if someone’s condition is hopeless. But what is an extraordinary measure? And what is hopeless? If there’s a 10 percent chance a treatment will extend someone’s life by six months, is that good enough? Should an end-stage Alzheimer’s patient, whose life is greatly diminished but still has moments of joy, be given antibiotics to treat an infection that otherwise might bring a quicker end?

There are other problems. People change their minds after they write their living wills but don’t update them. Many living wills never make it to the bedside, left in a file cabinet or safety deposit box. And family and doctors often do a poor job of deciphering the patient’s wishes even when they have the wills in hand.

“Most people believe that a living will can be written that is effective and that genuinely conveys real information that can be used in making decisions, and that is not true,” said Carl Schneider, an expert in medical law at the University of Michigan Law School.

Since 1990, hospitals have been required in most states to offer all patients the chance to fill out a living will. But despite the law and despite several high-profile end-of-life legal disputes, most people still don’t have living wills.

“Enough. The living will has failed, and it is time to say so,” Schneider and colleague Angela Fagerlin wrote in a bioethics journal, the Hastings Center Report.

A better end-of-life option, experts say, is to give a trusted loved one the legal authority to make decisions if you become incapacitated. Two large studies found more than 70 percent of people would prefer that doctors rely on family rather than living wills in making decisions about their care.

This has its own problems – the designated decision-maker may not understand what the patient’s wishes are, and picking one person to make decisions could cause tension inside a family.

The stereotypical conflict, doctors and nurses say, comes when the adult child arrives from out of town feeling guilty he hasn’t spent more time with his parent and resists steps to end the parent’s life, even as a brother or sister who lives nearby says this is what the parent wanted.

Still, experts say designating someone to make decisions allows for more flexibility than living wills.

Another choice that holds less legal weight but perhaps more influence: thoughtful conversations in advance with your regular doctors.

“I sit with patients and ask them, not what treatments are important to you, but what defines quality of life,” Morrison said. “What would be a fate worse than death for you? What makes life worth living?”

The questions have become more acute as medical advances have given doctors unprecedented ways to prolong life. At the same time, a series of high-profile legal disputes over whether to end medical treatment have prompted more people to consider putting their wishes explicitly into writing.

This fall, the Florida Supreme Court struck down a law that reconnected the feeding tube of Terri Schiavo, 40, who was severely brain damaged 14 years ago. The dispute pits her husband, who says she wouldn’t want to live this way, against her parents, who want to keep her alive. Schiavo did not have a living will.

But doctors don’t always follow a patient’s instructions even when they are clear. Sometimes the document doesn’t accompany the patient. Or the doctor fears litigation or simply disagrees with a patient’s wishes.

Naomi Naierman, president of the American Hospice Foundation, acknowledged there can be problems with living wills and other end-of-life plans. But doing nothing is far worse, she said.

“You’re only leaving a mess behind if you don’t have directives for when you cannot speak on your own behalf,” said Naierman, whose organization promotes living wills. “It’s a terrible burden to leave with the family.”