Dynamic concurrent partnership networks incorporating demography.

We introduce a population model that incorporates From a mathematical point of view we deal with continuous-time Markov chains at the individual level, with the interaction between individuals captured by a global variable describing opportunities for new partnerships. We show that for large time a stationary distribution is attained and we deduce various statistical features of that distribution, with particular attention for concurrency, i.e. the overlap in time of multiple partnerships of one and the same individual. Our ultimate motivation is to model the spread of sexually transmitted infections in the population, for which the present paper serves as a prelude.

Cost-effectiveness of prenatal screening for thalassaemia in Hong Kong.

OBJECTIVES: To determine the cost effectiveness of a universal prenatal screening program for alpha- and beta-thalassaemia. METHODS: We retrospectively reviewed our program from 1998 to 2002, and calculated the direct and indirect costs of various components. RESULTS: 18,936 women were screened at our prenatal clinic and 153 couples were subsequently referred to our Prenatal Diagnostic Centre for counselling and further investigations. In addition, there were 238 tertiary referrals and 157 self-referrals. After investigations, 84 fetuses were at risk of beta-thalassaemia major/beta-E thalassaemia, 19 of them were affected and 18 were aborted. The total expenditure on our program (HK 10.0 million dollars) would be less than the postnatal service costs (HK 40.4 million dollars) for 18beta-thalassaemia major fetuses if they were born. Of 361 women at risk of carrying a homozygous alpha0-thalassaemia fetus, 311 (86.2%) opted for the indirect approach (using serial ultrasound examinations to exclude Hb Bart's disease), and 76 (24.5%) subsequently underwent an invasive test for a definitive diagnosis. The sensitivity and false positive rate of this indirect approach was 100.0% and 2.9% respectively. CONCLUSION: It is cost effective to run a universal prenatal screening program in an area where both beta-thalassaemia and alpha-thalassaemia are prevalent. The indirect approach can effectively avoid an invasive test in unaffected pregnancies.

Repair of a pseudarthrosis of the lumbar spine. A functional outcome study.

Eighty-six patients had a total of eighty-eight primary attempts at repair of a pseudarthrosis that had developed after a localized arthrodesis in the lumbar spine. A follow-up questionnaire was sent to all patients at a mean of fifty-one months (range, twenty-five to seventy-eight months) after the operation; seventy-two patients (84 percent) completed to questionnaire. A solid fusion was ultimately achieved after the treatment of eighty-one (94 percent) of the eighty-six pseudarthroses for which radiographic data were available. With the numbers available, we could find no significant association between a solid fusion and the patient's age, gender, body-mass index, return to work, or outcome score. Despite the high rate of fusion after the index repair and subsequent procedures, only nineteen (26 percent) of the seventy-two patients who completed the questionnaire eventually had a good or excellent outcome. Seven (10 percent) had an excellent result (90 to 100 points), twelve (17 percent) had a good result (70 to 89 points), fourteen (19 percent) had a fair result (50 to 69 points), and thirty-nine (54 percent) had a poor result (less than 50 points). Nevertheless, fifty-one patients (71 percent) reported that the operation led to some improvement, and fifty-five (76 percent) said that they would have the operation again if the circumstances were similar to those before the repair of the pseudarthrosis. Thirty-four of the seventy-two patients were smokers and thirty-eight were non-smokers at the time of the operation. There was a negative linear association between the outcome scores and the number of pack-years (p = 0.02). Cessation of smoking before the operation positively affected the outcome; the patients who had stopped smoking had a mean outcome score of 65 points, compared with 45 points for those who had not stopped (p = 0.03). Patients who had stopped smoking were also more likely to return to work full time (p < 0.001). At the latest follow-up evaluation, twenty of the seventy-two patients had returned to full-time employment. Patients who had been receiving Workers' Compensation at the time of the operation generally did poorly on the outcome questionnaire, but, with the numbers available, they did not have a significantly different rate of solid fusion than patients who had not been receiving Workers' Compensation. Also, the outcome score and the rate of fusion were nor significantly affected by age or by obesity.