Dmitry Kovtun, who met with Litvinenko in a London hotel shortly before the ex-FSB officer was hospitalized with symptoms of radioactive poisoning, is himself undergoing tests for radiation at a Moscow hospital. Russian prosecutors earlier said Kovtun had been diagnosed with radiation poisoning.
"I am still going through some medical checks," Kovtun told RIA Novosti, adding that he feels "stable."
Litvinenko died on November 23, leaving an alleged deathbed statement implicating Russian President Vladimir Putin in his death. Doctors detected a lethal dose of the radioactive substance polonium-210 in his body.
A week ago, police in the German city of Hamburg found traces of polonium-210 in locations visited by Kovtun before his trip to London, and launched a criminal investigation into his possible polonium smuggling.
The agent-turned-businessman has been questioned by investigators probing the case twice.
He has repeatedly denied any involvement in the death of the outspoken Kremlin critic with ties to exiled Russian oligarch Boris Berezovsky, insisting that he is himself a victim.
Tests of Kovtun's ex-wife and his two children have found no traces of contamination.