The agency found "extremely high levels of bromide in plasma sampled from several affected patients" in the African country, the agency said in a statement.
Over 390 cases of a mysterious disease have been identified since the first case was reported in October, the WHO said.
The patients, more than half of whom are children under 15 years old, were being treated at a hospital in Cacuaco, in the north, the agency said.
The WHO and Angolan health experts investigating the disease suspected the cause was toxicological, given that symptoms included extreme drowsiness and an inability to walk unaided.
The blood tests were done in a laboratory in Munich, Germany, and were to be checked by a British laboratory, which was also testing food and environmental samples, it said.
WHO officials said Thursday they would not comment further until Angola's Health Ministry made an announcement.
Bromide is a chemical compound that is found in some forms naturally, for example in sea water, but that in other forms can be toxic. Severe poisoning can result in seizures, coma and death.