SEARCHING FOR A DONOR; BOY BATTLES TIME, DISEASE IN HUNT FOR A TRANSPLANT.

Byline: Kevin F. Sherry Daily News Staff Writer

Shannon Smith gives thanks every morning for having another day with her 3-year-old son, Steven. And she gives thanks again before bed for the precious day they had together.

Now her prayers are focused on the hope that a donor will step forward to give genetically matched bone marrow that could save the life of her son.

``I try to stay really positive,'' Smith said. ``If you think negative, then negative happens.''

Steven suffers from aplastic anemia, which renders him unable to produce bone marrow, the spongy tissue that manufactures oxygen-carrying red blood cells, infection-fighting white blood cells and blood-clotting platelets.

Last week, he was set to get a bone-marrow transplant from his 6-month-old sister, Serena, but a final genetic test revealed that the siblings were incompatible, putting the family's hopes for Steven's recovery on hold.

Now, doctors have used national and international marrow registries to identify three potential donors, but there is no guarantee they will come forward for Steven.

``If they get on the registry, I pray they're willing to go the full step,'' Smith said. ``I just pray that it's someone who's really willing to do it.''

Steven's dad, Scott Koss, a landscape contractor, said the toughest part is worrying about Steven and dealing with the medical questions that come up.

``I try to run a business and deal with this, and there's just one word for it: stressful,'' he said.

``This is not a common disease, so it's hard to get straight answers and make decisions day after day,'' he said. ``You don't know what's going to happen.''

Dr. Madeline Marcus, a fellow in pediatric hematology and oncology at UCLA Medical Center, where Steven is being treated, said the compatibility of the potential donors is being analyzed.

``We're working them all up,'' Marcus said. ``Our hope is that we'd be able to transplant him by the end of the year.''

A patient who gets transfusions from a sibling that match on all six criteria used have a better than 90 percent chance of survival, Marcus said. That percentage drops when the donor is not related and with each blood transfusion.

Meanwhile, Koss and Smith try to keep something like a normal life together but must take care because of Steven's weakened immune system and susceptibility to cuts and bruises.

Last week, Steven visited his neighborhood fire station, where he sat in the engines and played with a fire hose.

``All it takes is one fall and he can start bleeding internally,'' Smith said. ``All he knows is that mom and dad keep nagging him.

``He knows he's going into the hospital sometime. I want him to keep his spirit because he's got a great spirit and love for life.''

Doctors diagnosed Steven with aplastic anemia on May 23, 1996, a day after his second birthday. He has received 30 transfusions of platelets and red blood cells, but doctors are reluctant to give him additional transfusions because each procedure reduces the chance of a successful transplant, Marcus said.

``Time is his worst enemy right now,'' the doctor said.

What causes aplastic anemia is a mystery. Doctors term it an environmental disease caused by unknown toxins.

The National Marrow Donor Program began 11 years ago and now has the marrow type of more than 3.7 million people on file. About 10,000 people die every year who could be saved by a marrow transplant, said Lynnette Chandler, regional coordinator of Children for Tomorrow, the local marrow registry.

Chandler founded Children for Tomorrow six months after the national registry began to help search for a marrow donor for her 18-year-old daughter, Amanda, who suffers from Fanconi's anemia, a genetic form of Steven's disease.

For Smith and Koss, the waiting goes on.

``I'm praying,'' she said, ``and still enjoying every day and every moment with my son.''

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PHOTO (color) Stephen Koss, shown with his sister Serena, is awaiting a donor.