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Serum cystatin C has been suggested as a new marker of GFR. For the introduction of this marker into clinical use a rapid and automated method is required. We have developed and validated an assay for serum cystatin C using latex particle-enhanced immunoturbidimetry. Intra- and inter-assay precision were < 3% and < 5% across the assay range. Analytical recovery was 93 +/- 3.8% and no lack of parallelism was demonstrated. Regression analysis of a method comparison with an enzyme-enhanced radial-immunodiffusion method, gave PETIA = 0.074 + 0.93 x SRID, r = 0.98, N = 100. Inter-assay precision profiles showed cystatin C was measured with two-fold better precision than creatinine on the same analyzer. Cystatin C measurement was neither interfered with by icterus nor by hemolysis. 1/cystatin C versus 1/creatinine concentrations gave r = 0.67, N = 469. Comparison of Cr EDTA GFR with 1/cystatin C and 1/creatinine gave r = 0.81 and 0.50, respectively, N = 206. Calculating diagnostic sensitivity for abnormal GFR showed cystatin C to be significantly (P < 0.05) more sensitive than creatinine (71.4 vs. 52.4%). Cystatin C measurement using PETIA technology can be automated on the same instruments used routinely for the measurement of creatinine and offers better analytical performance and probably improved clinical sensitivity as a screening test for early renal damage.

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It has been shown by Lippold (1952) and Bigland, Hutter & Lippold (1953) that the force of an isometric contraction of a mammalian muscle is proportional to the voltage-time integral of the electrical activity recorded from it. For practical use to be made of these observations, it is necessary to determine the limits within which this relationship holds.If, for instance, a voluntary contraction is to be maintained at a constant tension, it might be expected that more motor units would be recruited as the tension developed by fatigued units declined. Such an occurrence would alter, with the degree of fatigue, the proportionality between electrical activity and tension. Other factors in fatigue may well influence the relation, such as neuromuscular block (Brown & Burns, 1949) or the effect of activity on the size of action potentials (Brown & von Euler, 1938).The experiments to be reported here were designed to investigate the relation between the electrical activity and the tension of isometrically contracting human muscle under conditions of fatigue. METHODSThe tension in the calf muscles of human subjects was measured as described before (Lippold, Naylor & Treadwell, 1952). Action potentials were recorded by means of silver suction electrodes placed over the medial border of soleus muscle, where this is subcutaneous, and were integrated electronically, the integral being displayed by a Dekatron scaling unit. Calibration confirmed that the count in 5 sec was proportional to the input voltage, within the range utilized in the experiment. A parallel channel recorded the electromyogram and its integral (as pips) on moving paper to enable a subsequent check to be made. RESULTSSubjects were first required to make 5 sec periods of isometric contraction at a series of constant tensions. These were usually in steps of approximately 3 % over a range of 9-45 % of the maximum voluntary strength. During each 5 sec period the electrical activity was recorded. After this procedure a con-

The program for in vitro fertilization at Bourn Hall began in October 1980. Various types of infertility have been treated during this time using the natural menstrual cycle or stimulation of follicular growth with antiestrogens and gonadotrophins. Follicular growth and maturation are assayed by urinary estrogens and LH, monitored regularly during the later follicular stage. Many patients had an endogenous LH surge; others needed an injection of HCG to induce ovulation. All oocytes were recovered by laparoscopy. Wide variations occurred in the time interval between the start of the LH surge and oocyte recovery and between oocyte recovery and insemination. Embryos taken between the one- and the eight-cell stage were replaced into their mother, no standard procedure being adopted for all patients. The results of all treatments including patient's responses during the follicular and luteal phases, oocyte recovery, fertilization, cleavage, replacement, implantation, abortion, and birth and the effect of factors such as replacing two or more embryos, maternal age, and previous obstetric history are described in detail. The incidence of implantation after embryo replacement improved from 16.5% initially to 30% currently. More than 118 babies have been born, and many pregnancies are continuing.