DOUBTS, discontent and embarrassment clouded Miss Patricia Leung's mind when she walked out of the Guangzhou customs office, running for a Kowloon-bound train with an AIDS-free certificate in her hand. The 33-year-old reporter said she was not surprised when she was picked for an AIDS test, since the tests had been widely-publicised in Hongkong during the past two weeks. It was on the return leg of her 17th trip to China within the past 10 months that she was picked for the test - a measure adopted by the Guangdong authorities in a bid to prevent the spread of AIDS. The story began when she handed her home-visit permit to a customs officer. He kept the travel document after a brief look, pointed to a room by the side of the counter and said: ''You have to be tested.'' ''I pretended not to hear the order and walked straight passed the counter, but in vain,'' Miss Leung said. Walking into the room, she found two nurses ready to conduct the test. ''Hygienic,'' she said, was not the right word to describe the room. One of the nurses, who wore a pair of gloves, took a blood sample while the other was busy taking down her personal details. It was not until Miss Leung saw two boxes on the floor - one apparently for used needles, the other for new ones - that she asked whether the needles were to be reused. ''How come you still view China as such a backward country?'' asked one of the nurses. ''Are we so backward as to reuse the needles?'' After the test Miss Leung was given a plastic syringe without a needle. The needle was thrown into one of the boxes on the floor. She was then escorted to a table where about 20 people were queuing for their AIDS-free certificates. She was asked to return the syringe and pay $160. While she waited, an argument broke out between an officer and a middle-aged traveller concerning the cotton balls that had been used to cover their fingers after the test. ''The man was astonished when an officer told us to leave the blood-stained cotton balls on the floor because they lacked a litter bin,'' Miss Leung said. She waited for about 20 minutes before she received her travel document and a certificate saying she was free from the HIV virus and AIDS. ''I don't know why they conduct the test on travellers who are leaving the country and I have no idea of what they would do if they found an HIV carrier,'' Miss Leung said.