Diabetes and dental caries in US adults: an analysis of data from the National Health and Nutrition Examination Survey, 2011-2018.

OBJECTIVES: To determine the relationship between diabetes and dental caries among US adults participating in the 2011-2018 National Health and Nutrition Examination Survey (NHANES). BASIC RESEARCH DESIGN: The NHANES was a cross-sectional study including clinical assessments, laboratory analysis, and interviews. The sample included 16,635 participants aged 20 years and older that represent 187,596,215 individuals in the US in a probability weighted sample. Outcome variables included overall total caries score (or number of decayed, missing, filled permanent teeth - DMFT index) and the presence of caries. Bivariate analysis, Poisson regression for total caries score, logistic regression for the presence of caries were used for analysis. RESULTS: Controlling for covariates, multiple Poisson regression revealed that total DMFT scores were associated with diabetes status (adjusted relative risk ratio (RR)controlled diabetes = 1.13, RRuncontrolled diabetes = 1.18; p⟨0.001), no college education, female sex, white race, elderly (≥ 65 years), cigarette smoking, obesity, yearly dental visits, seeing a dentist only for treatment. Similarly, multiple logistic regression shows that the odds of adults with diabetes having dental caries were higher than among those without diabetes (adjusted risk ratio (OR)controlled diabetes = 1.84, ORuncontrolled diabetes = 1.87; p⟨0.05). CONCLUSIONS: Diabetes was associated with a higher caries score and a greater risk for dental caries among US adults.

A Dynamic Three-Dimensional Air Pollution Exposure Model for Hong Kong.

INTRODUCTION: High-density high-rise cities have become a more prominent feature globally. Air quality is a significant public health risk in many of these cities. There is a need to better understand the extent to which vertical variation in air pollution and population mobility in such cities affect exposure and exposure-response relationships in epidemiological studies. METHODS: We used a novel strategy to execute a staged model development that incorporated horizontal and vertical pollutant dispersion, building infiltration, and population mobility patterns in estimating traffic-related air pollution (TRAP) exposures in the Hong Kong Special Administrative Region (HK SAR).Two street-level spatial monitoring campaigns were undertaken to facilitate the creation of a two-dimensional land-use regression (LUR) model. A network of approximately 100 passive nitric oxide-nitrogen dioxide (NO-NO2) monitors was deployed for two-week periods during the cool and warm seasons. Sampling locations were selected based on population and road network density with a range of physical and geographical characteristics represented. Eight sets of portable monitors for black carbon (BC) and particulate matter ≤2.5 µm in aerodynamic diameter (PM2.5) were rotated so as to be deployed at 80 locations for a 24-hour period. Land-use, geographical, and emissions layers were combined with the spatial monitoring campaign results to create spatiotemporal exposure models.Vertical air pollution monitoring was carried out at six strategic locations for two weeks in the warm season and two weeks in the cool season. Continuous measurements were carried out at four different heights of a residential building and on both sides of a street canyon. The heights ranged from as close to street level as practically possible up to a maximum of 50 meters (i.e., below the 20th floor). Paired indoor monitoring was included to allow the calculation of infiltration coefficients to feed into the dynamic component of the exposure model.The final phase of model development addressed population mobility. A population-representative travel behavior survey (n = 89,358) was used to produce the dynamic component of the model, with time-weighted exposure estimates split between home and work or school. Transport microenvironment exposures were taken from published literature. Time-activity exposure estimates were split by age, sex, and employment status.Development of the exposure model in distinct packages allowed the application of a staged approach to an existing cohort data set. Mortality risk estimates for an elderly cohort of 66,000 Hong Kong residents were calculated using increasing exposure model complexity. RESULTS: The street-level (2-dimensional [2D]) LUR modeling captured important spatial parameters and represented spatial patterns of air quality in Hong Kong that were consistent with the literature. Higher concentrations of gaseous pollutants were centered in Kowloon and the northern region of Hong Kong Island. PM2.5 and BC predictions exhibited a north-south/west-east gradient, with higher concentrations in the northwest due to regional transport of particulate pollutants from Mainland China. While the degree of explained variance of the models was in line with other LUR modeling efforts in Asia, R2 values ranged from 0.46 (NO2) to 0.59 (PM2.5).Exponential decay rates (k) were calculated at each monitoring location. While it was clear that k values were higher during the warm season than the cool season, no robust patterns were identified relating to the canyon physical parameters. Therefore, a single decay rate was used for each pollutant across the whole region for derivation of the 3-dimensional (3D) exposure layer (k = 0.004 and 0.012 for PM2.5 and BC, respectively). An alternative decay profile that capped decay at 20 meters above street level was proposed and evaluated. The electrochemical sensors deployed during the canyon campaigns did not exhibit the degree of interunit precision necessary to detect vertical variations in gaseous pollutants, and these results were excluded from the study.We found that values of the median infiltration efficiencies (Finf) for both BC and PM2.5 were especially high during the cool season (91%). Finf values were somewhat lower during the warm season (81% and 88% for PM2.5 and BC, respectively), and we found a significant negative correlation between air conditioning use and Finf. The Finf for a mechanically ventilated office building was 45% and 40% during the cool and warm seasons, respectively.Dynamic exposure estimates were compared against home outdoor estimates. As expected, the addition of an indoor component decreased time-weighted exposure estimates, which were balanced out to some extent by the inclusion of transport microenvironments. Overall, mean time-weighted exposures for the full dynamic model were around 20% lower than home outdoor estimates.Higher levels of exposures were found with working adults and students than for those neither in work nor study. This was due to the increased mobility of people going to work or school. The exposures to PM2.5, BC, and NO2 were, respectively, 13%, 39%, and 14% higher for people who were under age 18, compared with people who were 65 or older. Exposure estimates for the female population were approximately 4% lower.The availability of an existing cohort data set of elderly Hong Kong residents (n = 66,820) facilitated the calculation and comparison of mortality risk estimates for the different exposure models.Overall, results indicated that the application of exposure estimates that incorporated infiltration, vertical, and to a lesser extent, dynamic components resulted in higher hazard ratios (HRs) than the standard street-level model and increased the number of significant associations with all-natural-cause, cardiovascular, and respiratory mortality outcomes. CONCLUSIONS: The results from the study provided the first evidence that considering air pollution exposure in a dynamic 3D landscape would benefit epidemiological studies. Higher HRs and a greater number of significant associations were found between mortality and pollutant exposures that would not have been found had standard 2D exposure models been used. Dynamic models can also identify differential exposures between population subtypes (e.g., students and working adults; those neither in work nor study).Improved urban building design appears to be stimulating the dispersion of local TRAP in street canyons. Conversely, Finf values found in naturally ventilated buildings were high, and residences provided little protection from ambient air pollution.We have demonstrated that the creation of effective advanced exposure models is possible in Asian cities without an undue burden on resources. We recommend that vertical exposure patterns be incorporated in future epidemiological studies in high-rise cities where the floor of residence is recorded in health record data.

Weather inference and daily demand for emergency ambulance services.

AIM: To examine weather effects on the daily demand for ambulance services in Hong Kong. METHODS: Over 6 million cases of emergency attendance from May 2006 through April 2009 (3 years) were obtained from the Hospital Authority in Hong Kong. These cases were further stratified by age, triage levels, hospital admission status, comprehensive social security assistance (CSSA) recipients and gender. The stratification was used to correlate against weather factors to assess the dependency of these variables and their effects on the daily number of ambulance calls. Adjusted-R(2) values obtained from the regression analysis were used as a measure for evaluating predictability. RESULTS: The adjusted-R(2) of emergency cases by age groups showed proportional correlation with weather factors, which was more significant in older patients (0.76, p<0.01) than young patients (0.10, p<0.05). Furthermore, patients with more severe conditions were shown to have a higher adjusted-R(2) (0.63, p<0.05 for critical as opposed to 0 for non-urgent patients). Weather effects were also found more significant in women (0.50, p<0.01) and CSSA recipients (0.54, p<0.01) when compared against their corresponding reference groups (respectively men at 0.46, p<0.01 and non-CSSA recipients at 0.45, p<0.01). Moreover, average temperature appeared to be a major weather effect. CONCLUSIONS: The presence of strong weather effects among different target groups indicates possibility for the development of a short-term forecast system of daily ambulance demand using weather variables. The availability of such a forecast system would render more effective deployment of the ambulance services to meet the unexpected increase in service demands.

Global association between satellite-derived nitrogen dioxide (NO2) and lockdown policies under the COVID-19 pandemic.

The COVID-19 pandemic has severely affected various aspects of life, at different levels and in different countries on almost every continent. In response, many countries have closed their borders and imposed lockdown policies, possibly bringing benefits to people's health with significantly less emission from air pollutants. Currently, most studies or reports are based on local observations at the city or country level. There remains a lack of systematic understanding of the impacts of different lockdown policies on the air quality from a global perspective. This study investigates the impacts of COVID-19 pandemic towards global air quality through examining global nitrogen dioxide (NO2) dynamics from satellite observations between 1 January and 30 April 2020. We used the Apriori algorithm, an unsupervised machine learning method, to investigate the association among confirmed cases of COVID-19, NO2 column density, and the lockdown policies in 187 countries. The findings based on weekly data revealed that countries with new cases adopted various lockdown policies to stop or prevent the virus from spreading whereas those without tended to adopt a wait-and-see attitude without enforcing lockdown policies. Interestingly, decreasing NO2 concentration due to lockdown was associated with international travel controls but not with public transport closure. Increasing NO2 concentration was associated with the "business as usual" strategy as evident from North America and Europe during the early days of COVID-19 outbreak (late January to early February 2020), as well as in recent days (in late April) after many countries have started to resume economic activities. This study enriches our understanding of the heterogeneous patterns of global associations among the COVID-19 spreading, lockdown policies and their environmental impacts on NO2 dynamics.

Posterior mediastinal abscess in a hemodialysis patient - a rare but life-threatening complication of Staphylococcus bacteremia.

Mediastinal abscess is a rare but life-threatening disease with grave prognosis if not recognized in time. The diagnosis of this entity is hampered by its nonspecific presentation. Thus, a high index of clinical suspicion is one of the keystones in the successful management of these patients. Currently, mediastinitis occurred mostly as a complication post cardiovascular or thoracic surgical procedures. Rarely, primary mediastinitis developed as a result of hematogenous spread. We report the case of primary mediastinitis in a renal failure patient who had Staphylococcus aureus bacteremia. Despite antibiotic treatment, she had massive hemoptysis and eventually succumbed to the disease. To our knowledge, this is the first report of Staphylococcus aureus bacteremia complicating mediastinal abscess in a dialysis patient with a native arteriovenous shunt. Our experience in this case suggests that dialysis patients are under constant risk of staphylococcal infection due to regular venous puncture. Measures should be taken to reduce the risk and staphylococcal infection should not be forgotten in any dialysis patient with fever of unknown origin.

BK virus infection in association with posttransplant urothelial carcinoma.

OBJECTIVE: BK virus infection after transplantation is known to cause graft failure but the association with malignancies is controversial. METHODS: BK virus workup was performed for kidney recipients in our center under conditions of hematuria or acute deterioration of graft function. We reviewed the history and reported our treatment and the disease course of three patients with BK virus later diagnosed with urothelial carcinoma. RESULTS: All three patients received kidneys from China with immunosuppression using a calcineurin inhibitor and monoclonal antibodies. Synchronous bladder and upper-tract tumors were treated with surgery followed by intravesical chemotherapies. We tapered the immunosuppressants and changed to a sirolimus-based regimen. Intravesical chemotherapy and concurrent chemoradiotherapy were performed to prevent recurrence. All three patients now have functional grafts. CONCLUSION: BK virus infection may lead to tumorigenesis. Besides decreasing immunosuppressants, we should be more alert to the detection of malignancies in BK virus-reactivated recipients. Early aggressive treatment may be curative, preserving functional grafts.

Spatial clustering of SARS in Hong Kong.

1. Geographic information system (GIS) can be applied during an acute infectious disease outbreak to reveal new geospatial information in addition to standard field epidemiological analyses. 2. When applied in real time during the onset and evolution of an epidemic, GIS can monitor and enhance understanding of the transmission dynamics of an infectious agent, thereby facilitating the design, implementation and evaluation of potential intervention strategies.

HLA class I antibodies in patients awaiting kidney transplantation and the association with renal graft survival.

The presence of alloantibodies against human leukocyte antigens (HLA) in the circulation of a transplant recipient shows a significant negative impact on the outcome of solid-organ transplantations. The aim of this study was to examine the impact on renal graft survival of various patterns of alloantibodies detected among patients awaiting kidney transplantation. Among more than 2000 patients awaiting kidney transplantations between July 1992 and March 2006, were 683 patients who displayed anti-HLA alloantibodies, 318 of whom were enrolled in this study. Each patient was followed for at least 9 months; the presence of HLA alloantibodies was checked every 3 months by an enzyme-linked immunosorbent assay. Among these 318 patients, 55 patients underwent kidney transplantations. Their median follow-up time was 69 (range, 9-129) months, including 267 (84%) who displayed persistent class I HLA alloantibodies. The intermittent presence of class I HLA alloantibodies was seen in 20 (6.3%) patients. Serum class I HLA antibodies which was positive at first then became undetectable in 4 (1.3%) patients. Three (0.9%) patients were unsensitized at first and then developed class I HLA alloantibodies later; & 24 (7.5%) patients had class I HLA alloantibodies only once during the follow-up period. Among these patients, 55 patients received renal transplantations. The median survival time was shortest in the patients with persistent class I HLA alloantibodies (59.9 months) and longest among patients who were positive at first and then became negative thereafter or in whom class I HLA alloantibodies was detected only once (132 months). There was a significant difference in graft survival times between patients who had persistent HLA alloantibodies and those in whom to have class I HLA alloantibodies were detected only once (P < .05). In this study, the persistent presence of class I HLA alloantibodies among pretransplantation patients was associated with poorer renal graft outcomes. Surveys of various patterns of sensitization to class I HLA antigen may help us to perform risk stratification. High-risk patients may need more aggressive approaches to deplete antibody or complement levels.

Sirolimus conversion experience in a single center.

OBJECTIVE: One major cause of graft loss is chronic allograft nephropathy (CAN), which may relate to calcineurin inhibitors (CNIs). We converted CAN cases from CNIs to sirolimus and observed the outcomes. METHOD: From January 2004 to August 2007, there were 28 kidney recipients in our center with creeping creatinine levels compatible with CAN. We started sirolimus at 2 mg/d and reduced the CNIs gradually. Sirolimus trough levels were kept between 5 and 8 ng/mL. Mycophenolic acid was cut in half; there was no adjustment on prednisolone dose. RESULTS: The mean switch time was 47.3 months after transplantation. One case discontinued sirolimus due to severe drug-induced pneumonitis. Twelve of the 27 (45%) patients showed improvements in graft function. The most frequent complications were anemia (13/28), hyperlipidemia (13/28), and pneumonitis (4/28). A baseline serum creatinine level less than 2.2 mg/dL seemed to forecast a response to sirolimus conversion. Most of the graft functional improvement occurred within 6 months after the switch. No graft or patient loss was encountered. CONCLUSION: Our experience suggested that 45% of patients with sirolimus conversion showed improved graft function. Among patients within 1 year after transplantation, those with a creatinine level less than 2.2 mg/dL, no proteinuria, and no hyperlipidemia seemed to be better candidates for Sirolimus conversion.

Sirolimus used during pregnancy in a living related renal transplant recipient: a case report.

OBJECTIVES: The majority of pregnancies after transplantation reported in the literature occurred in patients treated with a combination of calcineurin inhibitors, prednisolone, and azathioprine. There is little experience with newer drugs. We report a successful pregnancy in a kidney recipient with exposure to sirolimus-based immunosuppression. METHODS: We describe a case of successful delivery in a 30-year-old woman who became pregnant 1 year and 8 months after a living related renal transplantation. She received sirolimus, cyclosporine, and prednisolone before conception and during the first and second trimesters of gestation. RESULTS: The female recipient received sirolimus in combination with cyclosporine and prednisolone. During follow-up, her serum creatinine values were stable with pregnancy occurring at 1 year and 8 months after transplantation. At 27 gestational weeks, sirolimus was discontinued and she was maintained on cyclosporine and prednisolone. There were no signs or symptoms of graft rejection. A Cesarean section was performed at 39 weeks of gestation to deliver a healthy, 2994-g, Apgar 10, male infant. The renal function of the female recipient continued to be stable after delivery. CONCLUSION: To date, pregnancies in renal transplant recipients are still considered high risk. The U.S. National Transplantation Pregnancy Registry (NTPR) has reported increased rates of maternal and fetal complications. There have been no live births reported to the NTPR about female recipients exposed to sirolimus throughout gestation. We report a live birth without a structural defects with successful delivery after sirolimus use during the first and second trimesters of gestation.

Osteoporosis after kidney transplantation: preliminary report from a single center.

OBJECTIVE: One of the major adverse effects of kidney transplantation is osteoporosis, which is mainly related to steroid use. Only limited data are available on calcitonin therapy for posttransplantation osteoporosis. METHOD: From March 2007 to August 2007, 67 kidney recipients agreed to enter this study. Dual energy X-ray absorptiometry (DEXA) was performed to evaluate bone mineral density (BMD) in the lumbar (L) spine and left femoral neck. We prescribed calcitonin nasal spray to osteoporosis patients (DEXA T < -2.5 SD) who agreed with the treatment. A second and a third DEXA were performed at 3-month subsequent intervals later to evaluate the therapeutic effects. RESULTS: The incidence of osteoporosis in our kidney recipients was 46.26% (31/67 patients). Osteopenia accounted for 38.81% (26/67 patients) and only 14.93% (10/67 patients) were normal. Calcitonin inhalation seemed to improve the BMD with 61% showing improvement on the second DEXA study in our preliminary data. CONCLUSION: Our preliminary data suggested that calcitonin may help to restore bone mass in kidney recipients with osteoporosis. Steroid elimination may prevent the onset of osteoporosis and might even enhance calcitonin efficacy. In the future we need a longer study period to confirm the results and compare it with the outcomes of bisphosphonates therapy.

Kidney ultrasound is useful tool in posttransplant follow-up.

OBJECTIVES: Renal sonography has been an important imaging tool in surveys of kidney diseases. We reviewed our experience in the finding and management of asymptomatic patients who underwent renal transplantation. METHODS: We performed baseline graft and native kidney sonography after renal transplantation, as well as annually even if the patient was asymptomatic. At the end of 2004, a total of 326 transplant cases had been annually surveyed. If the findings were positive, they were compared with previous data to determine the need for further examinations and management. RESULTS: The native kidneys of 105 patients were abnormal. Cysts were detected in 71 cases, 23 of which were bilateral. Stones were found in 15 cases. Polycystic kidney disease was identified in 5. The findings in these 91 patients were the same as before. Moderate hydronephrosis was observed in 14 cases. Nine had native ureteral cancer and underwent nephroureterectomy. Ureteral stricture was found in the other 5 patients. Forty-five grafts were abnormal. Thirty-one showed hydronephrosis and 2 underwent ureteral reimplantation. Asymptomatic stones were found in 2. A new single renal cyst was found in 2 cases; and multiple cysts in one other. Elevated RI on color Doppler was discovered in 12 patients, 4 of whom lost their grafts this year. Serum creatinine values of 6.9 and 2.2 mg/dL were observed in 2 patients. CONCLUSIONS: Renal sonography screening is useful not only for the graft but also for the native kidney. Hydronephrosis is an important finding. The high possibility of urothelial malignancy should be expected, requiring further examination and sequential follow-up. Elevated RI is a clue to predict graft outcome; rapid deterioration was observed within months.

Bovine fetus-specific serum proteins. Purification and characterization of alpha1-fetoprotein and immunological identification of alpha2- and beta-fetoproteins.

Bovine alpha1-fetoprotein was isolated from fetal calf serum by successive procedures of concanavalin A-Sepharose chromatography, DEAE-Sephadex chromatography, SP-Sephadex chromatography and preparative disc polyacrylamide gel electrophoresis. The bovine alpha1-fetoprotein preparation was considered homogeneous by several physicochemical and immunochemical criteria. Bovine alpha1-fetoprotein has a molecular weight of 68 000 with an amino acid composiotn similar to that of other mammalian alhpa1-fetoprotein. In addition, bovine alpha1-fetoprotein was shown to exist as two distinct variants on the basis of carbohydrate heterogeneity. alpha2-Fetoprotein and a new beta-fetoprotein were immunologically identified in fetal calf serum. These fetoproteins, like alpha1-fetoprotein, were not detectable in non-pregnant cow serum by immunoelectrophoresis.

Immunochemical purification and characterization of ovine alpha-fetoprotein.

Ovine alpha-fetoprotein was successfully isolated from fetal sheep serum by using rabbit anti-ovine alpha-fetoprotein linked to an agarose immunoadsorbent column. Antibody used in this affinity chromatography column was produced by immunizing a rabbit with highly purified alpha-fetoprotein-antibody complex to yield a monospecific antiserum to ovine alpha-fetoprotein. Following affinity chromatography, alpha-fetoprotein was further purified by preparative polyacrylamide disc gel electrophoresis ultimately yielding a 105-fold purification. The purified alpha-fetoprotein was homogeneous on analytical polyacrylamide disc gel electrophoresis. Ovine alpha-fetoprotein was found to be immunochemically related to human alpha-fetoprotein and to exhibit a molecular weight and amino acid composition similar to other mammalian alpha-fetoproteins.

Interleukin-11 reduces renal injury and glomerular NF-kappa B activity in murine experimental glomerulonephritis.

BACKGROUND/AIMS: There is now considerable evidence implicating T cells and macrophages in glomerular injury in crescentic glomerulonephritis. Recently, it has been shown that interleukin-11 (IL-11) has an immune modulatory function through its effect on both macrophages and T cells. We, therefore, examined the therapeutic effect of IL-11 in a murine model of experimental glomerulonephritis. METHOD: Accelerated nephrotoxic nephritis was induced in C57BL/6 mice. IL-11 at a dose of 0.5 mg/kg/day (n = 10) in vehicle was given daily subcutaneously from the day of sensitization until day 14 after initiation of glomerulonephritis. Control mice (n = 10) received injection of vehicle alone with the same schedule. RESULTS: IL-11 treatment markedly decreased albuminuria (6.2 +/- 1.9 vs. 18.2 +/- 4.5 mg/day, p < 0.05), the number of glomerular macrophages (1.1 +/- 0.2 vs. 1.7 +/- 0.3 cells/glomerular cross-section, p < 0.05) and glomerular fibrin deposition (fibrin score 0.9 +/- 0.3 vs. 2 +/- 0.3, p < 0.05). There was no difference in the glomerular T cell numbers between the IL-11-treated and the vehicle group. Glomerular NF-kappaB activity was markedly suppressed by 75% in the treated group (p = 0.0015). CONCLUSION: In this study, we provide the first in vivo evidence that IL-11 treatment decreases glomerular NF-kappaB activity and reduces renal injury in experimental glomerulonephritis.

Radioimmunoassay of ovine alpha-fetoprotein.

Highly purified ovine alpha-fetoprotein (AFP) was used both for radioisotope labelling and as the reference standard in the double antibody radioimmunoassay of ovine AFP. The sensitivity of the assay is 2 ng/ml which is about 8000 times more sensitive than radialimmunodiffusion assay. The assay is of sufficient sensitivity to quantitate AFP in normal adult sheep serum, pregnancy serum, amniotic fluid and fetal lamb serum.

Understanding the spatial clustering of severe acute respiratory syndrome (SARS) in Hong Kong.

We applied cartographic and geostatistical methods in analyzing the patterns of disease spread during the 2003 severe acute respiratory syndrome (SARS) outbreak in Hong Kong using geographic information system (GIS) technology. We analyzed an integrated database that contained clinical and personal details on all 1,755 patients confirmed to have SARS from 15 February to 22 June 2003. Elementary mapping of disease occurrences in space and time simultaneously revealed the geographic extent of spread throughout the territory. Statistical surfaces created by the kernel method confirmed that SARS cases were highly clustered and identified distinct disease "hot spots." Contextual analysis of mean and standard deviation of different density classes indicated that the period from day 1 (18 February) through day 16 (6 March) was the prodrome of the epidemic, whereas days 86 (15 May) to 106 (4 June) marked the declining phase of the outbreak. Origin-and-destination plots showed the directional bias and radius of spread of superspreading events. Integration of GIS technology into routine field epidemiologic surveillance can offer a real-time quantitative method for identifying and tracking the geospatial spread of infectious diseases, as our experience with SARS has demonstrated.

A rapid radioimmunoassay for human alpha-fetoprotein.

Our previous double antibody radioimmunoassay for human AFP was run at 4 degrees C and required 48 h for completion. A method is described which permits the assay to be performed within 5 h due to an increased incubation temperature and the use of polyethylene glycol to precipitate the antigen-antibody complex. The modified radioimmunoassay was standardized against an international reference preparation of human AFP. Characteristics of this assay are discussed.