More than 100 parents took their children to Princess Margaret Hospital for kidney checks yesterday, fearing for their health after having fed them milk products they suspect were adulterated with the industrial chemical melamine. Melamine causes kidney stones and other renal problems. Four children on the mainland fed tainted baby formula have died and more than 100 have acute renal failure. Emily Tang, whose sons, aged eight and 10, had often drunk milk made by mainland suppliers Yili and Mengniu milk over the past two years, told of a frustrating time. Yili and Mengniu are among producers whose products have been found to contain melamine. 'We came [to the paediatrics and adolescent medicine clinic] at 10am, but the nurses did not tell us the procedures ... we had no idea what to do and where to go,' she said. 'Many parents are frustrated. They have waited for a long time without having lunch.' Ms Tang also said it was unfair that some children had been given checks at the paediatric unit without referrals. She said she had been told by medical staff that children had to first obtain referrals from either public hospital accident and emergency departments or public outpatient clinics. Secretary for Food and Health York Chow Yat-ngok said 18 outpatient clinics would check children from today. Dr Chow said: 'I offer my apology to parents who have taken their children for check-ups at Prince Margaret Hospital. I agree the arrangements are not satisfactory. We have underestimated the response of the public.' The Hospital Authority said the 18 clinics would give free check-ups to children under 12 who have developed symptoms after drinking suspect milk products. Each clinic can cope with 50 to 100 cases a day. The authority will also open seven assessment centres. Patients will be referred to them for further examination if necessary. Each centre can handle up to 40 cases a day. A three-year-old girl was treated at Princess Margaret Hospital on Saturday for a kidney stone after consuming Yili-brand milk for 15 months. That prompted parents to rush to the hospital for check-ups. Paediatric specialist Mark Chan Kwok-ho, who treated a 10-month-old Macau girl for kidney stones after she ate contaminated food, said prolonged fever and frequent mood swings could be a sign of colic caused by kidney stones.