Alexander C. Smith

Anti-Müllerian hormone: poor assay reproducibility in a large cohort of subjects suggests sample instability

STUDY QUESTION What is the variability of anti-Müllerian hormone (AMH) concentration in repeat samples from the same individual when using the Gen II assay and how do values compare to Gen I [Diagnostic Systems Ltd (DSL)] assay results? SUMMARY ANSWER The Gen II AMH assay displayed appreciable variability, which can be explained by sample instability. WHAT IS KNOWN ALREADY AMH is the primary predictor of ovarian performance and is used to tailor gonadatrophin dosage in cycles of IVF/ICSI and in other routine clinical settings. Thus, a robust, reproducible and sensitive method for AMH analysis is of paramount importance. The Beckman Coulter Gen II ELISA for AMH was introduced to replace earlier DSL and Immunotech assays. The performance of the Gen II assay has not previously been studied in a clinical setting. STUDY DESIGN, SIZE AND DURATION We studied an unselected group of 5007 women referred for fertility problems between 1 September 2008 and 25 October 2011; AMH was measured initially using the DSL AMH ELISA and subsequently using the Gen II assay. AMH values in the two assays were compared using a regression model in log(AMH) with a quadratic adjustment for age. Additionally, women (n = 330) in whom AMH had been determined in different samples using both the DSL and Gen II assays (paired samples) identified and the difference in AMH levels between the DSL and Gen II assays was estimated using the age-adjusted regression analysis. A subset of 313 women had repeated AMH determinations (n = 646 samples) using the DSL assay and 87 women had repeated AMH determinations using the Gen II assay (n = 177 samples) were identified. A mixed effects model in log(AMH) was utilized to estimate the sample-to-sample (within-subject) coefficients of variation of AMH, adjusting for age. Laboratory experiments including sample stability at room temperature, linearity of dilution and storage conditions used anonymized samples. MAIN RESULTS AND THE ROLE OF CHANCE In clinical practice, Gen II AMH values were ∼20% lower than those generated using the DSL assay instead of the 40% increase predicted by the kit manufacturer. Both assays displayed high within-subject variability (Gen II assay CV = 59%, DSL assay CV = 32%). In the laboratory, AMH levels in serum from 48 subjects incubated at RT for up to 7 days increased progressively in the majority of samples (58% increase overall). Pre-dilution of serum prior to assay, gave AMH levels up to twice that found in the corresponding neat sample. Pre-mixing of serum with assay buffer prior to addition to the microtitre plate gave higher readings (72% overall) compared with sequential addition. Storage at -20°C for 5 days increased AMH levels by 23% compared with fresh samples. The statistical significance of results was assessed where appropriate. LIMITATIONS, REASONS FOR CAUTION The analysis of AMH levels is a retrospective study and therefore we cannot entirely rule out the existence of differences in referral practices or changes in the two populations. WIDER IMPLICATIONS OF THE FINDINGS Our data suggests that AMH may not be stable under some storage or assay conditions and this may be more pronounced with the Gen II assay. The published conversion factors between the Gen II and DSL assays appear to be inappropriate for routine clinical practice. Further studies are urgently required to confirm our observations and to determine the cause of the apparent instability. In the meantime, caution should be exercised in the interpretation of AMH levels in the clinical setting. CONFLICT OF INTEREST/STUDY FUNDING S. Roberts is supported by the NIHR Manchester Biomedical Research Centre.

Estimating rotational iron losses in an induction machine

We present an assessment of the effect of rotational losses on an induction machine. The assessment provides an estimate of the iron losses in an induction machine by three methods, all of which rely on the output data of a two-dimensional finite-element method: 1) calculating iron losses as if they were produced by a purely alternating field; 2) calculating the iron losses by adding the losses produced by the orthogonal components of the flux density, as if the losses produced by these components were independent phenomena; 3) applying a correction factor based on experimental data to improve the rotational loss calculation. The correction factor is a function of the peak flux density value and the ratio of the major to the minor axis of the flux density loci. The third method represents the main contribution of this paper to the field and is explained in detail. Finally, a discussion of the results addresses two aspects: the location where rotational fields occur, and their impact on the total loss calculation.

A transient induction motor model including saturation and deep bar effect

Transient cage induction motor models for use in inverter-fed drives and controllers are reviewed. A simple transient model is presented that includes rotor deep bar effect and magnetic saturation of the magnetising and rotor leakage flux paths. The improved model requires motor details in the form of simple impedance versus frequency characteristics which can be obtained from a variety of external sources. These can range typically from detailed steady-state finite-element solutions to simple experimental measurements. The model is verified experimentally using a 75 kW, 4 pole vector controlled AC motor drive.

Propulsion Drive Models for Full Electric Marine Propulsion Systems

Integrated full electric propulsion systems are being introduced across both civil and military marine sectors. Standard power systems analysis packages cover electrical and electromagnetic components, but have limited models of mechanical subsystems and their controllers. Hence electromechanical system interactions between the prime movers, power network and driven loads are poorly understood. This paper reviews available models of the propulsion drive system components: the power converter, motor, propeller and ship. Due to the wide range of time-constants in the system, reduced order models of the power converter are required. A new model using state-averaged models of the inverter and a hybrid model of the rectifier is developed to give an effective solution combining accuracy with speed of simulation and an appropriate interface to the electrical network model. Simulation results for a typical ship manoeuvre are presented.

The Manchester Acute Coronary Syndromes (MACS) decision rule for suspected cardiac chest pain: derivation and external validation

Objective We aimed to derive and validate a clinical decision rule (CDR) for suspected cardiac chest pain in the emergency department (ED). Incorporating information available at the time of first presentation, this CDR would effectively risk-stratify patients and immediately identify: (A) patients for whom hospitalisation may be safely avoided; and (B) high-risk patients, facilitating judicious use of resources. Methods In two sequential prospective observational cohort studies at heterogeneous centres, we included ED patients with suspected cardiac chest pain. We recorded clinical features and drew blood on arrival. The primary outcome was major adverse cardiac events (MACE) (death, prevalent or incident acute myocardial infarction, coronary revascularisation or new coronary stenosis >50%) within 30 days. The CDR was derived by logistic regression, considering reliable (κ>0.6) univariate predictors (p<0.05) for inclusion. Results In the derivation study (n=698) we derived a CDR including eight variables (high sensitivity troponin T; heart-type fatty acid binding protein; ECG ischaemia; diaphoresis observed; vomiting; pain radiation to right arm/shoulder; worsening angina; hypotension), which had a C-statistic of 0.95 (95% CI 0.93 to 0.97) implying near perfect diagnostic performance. On external validation (n=463) the CDR identified 27.0% of patients as ‘very low risk’ and potentially suitable for discharge from the ED. 0.0% of these patients had prevalent acute myocardial infarction and 1.6% developed MACE (n=2; both coronary stenoses without revascularisation). 9.9% of patients were classified as ‘high-risk’, 95.7% of whom developed MACE. Conclusions The Manchester Acute Coronary Syndromes (MACS) rule has the potential to safely reduce unnecessary hospital admissions and facilitate judicious use of high dependency resources.

Transient analysis of cage-induction motors using finite-elements

A method for the analysis of induction motor transients using the two-dimensional finite-element method is described. Unlike previously published work the method does not use an eddy-current formulation to account for the currents induced in the rotor bars. It relies on the decomposition of the rotor currents into harmonic distributions and the use of auxiliary circuit equations to take the time-stepping into account. The adoption of this technique yields some distinct advantages. Firstly, time-stepping is limited to the circuit model alone, thereby allowing an optimum step size to be selected at all times. Secondly, the use of a magnetostatic finite-element formulation makes it possible to use nonlinear solutions of comparatively low accuracy, thus bringing substantial savings in computer resources. Finally, the extensive use of circuit techniques also allows the effect of axial skew and spatially distributed airgap flux harmonics to be incorporated. >

Improved Energy Conversion for Doubly Fed Wind Generators

There is currently significant interest in offshore wind turbines up to 5 MW. One of the preferred options is doubly-fed slip-ring generators. Turbine manufacturers have proposed an operating scheme for low speeds that is claimed to improve the overall energy extraction from the wind. The aim of this paper therefore is to examine the performance benefits for this new operational mode on a small experimental rig, confirmed by simulation, and to provide a fundamental understanding of the differences in the generator operation in both modes. The paper develops analytical steady-state models to provide this insight and correlates the operating performance with a dynamic real-time generator control scheme and experimental results obtained from a laboratory test machine. A theoretical study on a 2 MW commercial turbine is also undertaken.

Improved cage rotor models for vector controlled induction motors

Variations in motor model parameters lead to the detuning of vector controllers which, in turn, cause a deterioration in performance, particularly during transient and field weakening operation. Parameter adaptation schemes seek to change the model parameters based on observed performance, but are still based around a simple model for the machine which is not adequately representative. Improved motor models, based on deeper phenomenological understanding of the machine offer a viable alternative. In this paper, the authors present a model which enables rotor leakage and main flux path saturation and deep-bar effects to be taken into account. The model is verified experimentally using a 7.5-kW 4-pole motor, mounted on a piezoelectric force table which enables transient torques to be measured. >

Design Concepts for High-Voltage Variable-Capacitance DC Generators

The transmission requirements of the next generation of offshore wind farms, such as the Round Two U.K. offshore development proposals, may rely on high-voltage direct current (HVDC) technology for at least a part of their power collection and transmission requirements. HVDC technology is particularly suited for the transmission of high powers through large lengths of submarine cables; however, its application is limited by the high cost of offshore ac-dc converter stations. This paper therefore investigates the feasibility of the direct generation of HVDC power using a novel generator topology, as an alternative solution to the generation-transmission requirements of large offshore wind farms. A variable-capacitance generator uses electrostatic fields to generate an HVDC output with a minimum of power conditioning and is based on a previous work that suggests system power densities comparable with the conventional generator-transformer-rectifier systems to be achievable.

High torque dense naval propulsion motors

This paper reviews the current state of marine electric propulsion systems and explains why these developments are economically and operationally desirable. The paper reviews the current fashion for multi-phase designs for the propulsion motors and specifically considers the benefits to be achieved by using a high phase number to excite a propulsion induction motor. The paper also examines the important issue of fault tolerance in multi-phase propulsion drives.