Development and evaluation of a kinetic diacetyl monoxime method for urine urea.

A kinetic assay for urine urea which employs the reaction with diacetylmonoxime at 37 degrees C and undiluted urine was developed. Reaction conditions were optimized using the Monarch centrifugal analyser to produce an assay with the linearity to 450 mmol/L urea, and CVs of 2.3% and 2.5% within-batch and 4.6% and 3.3% between-batch at urea concentrations of 75.9 and 162.7 mmol/L, respectively; recoveries of 98.5% to 99.5% were obtained with the addition of the equivalent of 95 and 190 mmol/L urea. Comparisons were made with one urine urea method employing diacetylmonoxime with dialysis and two enzymatic methods which all involve a sample pre-dilution stage. The urine samples used for the method comparisons did not contain preservatives. However, a range of urine preservatives and a selection of possible interfering metabolites were shown to have no significant effect on the recovery of urea in the assay.

Development and evaluation of the simultaneous determination of sweat sodium and chloride by ion-selective electrodes.

A method for the simultaneous determination of sweat sodium and chloride was developed and evaluated using a Monarch 2000 centrifugal analyser with ion-selective electrodes. In order to measure sodium and chloride in diluted sweat samples and sweat paper eluates an analytical working range of below 10 mmol/L was required. This was achieved by modification of the Monarch ISE software parameters for urine electrolytes together with the use of calibrators within this range. Evaluation of the method has included linearity, recovery and precision studies and the correlation between ion-selective electrode and flame photometry for sweat sodium and between ion-selective electrode and a continuous flow method for sweat chloride. Of the patients studied so far, there has been good discrimination in sweat electrolytes between those with and without cystic fibrosis.

Maternal serum alpha-fetoprotein screening for open neural tube defects in twin pregnancies.

Data on maternal serum alpha-fetoprotein (AFP) levels at 13-24 weeks' gestation in 46 twin pregnancies with open neural tube defects (22 with anencephaly, 24 with open spina bifida) and 169 unaffected twins were used to estimate the detection and false-positive rates associated with different cut-off levels. Using the conventional cut-off level of 2.5 multiples of the median (MoM) for unaffected singleton pregnancies of the same gestation and laboratory, the detection rate in twins was 99 per cent for anencephaly and 89 per cent for open spina bifida, with a false-positive rate of 30 per cent. Using a 5.0 MoM cut-off level to maintain a similar false-positive rate to that found among singleton pregnancies at 16-18 weeks' gestation (about 3 per cent), the detection rate was 83 per cent for anencephaly and 39 per cent for open spina bifida. Estimates are provided of the odds of having an affected twin pregnancy given a positive AFP result as well as the odds for individual women with a raised AFP level.

Measurement of plasma phenytoin by EMIT in the Monarch centrifugal analyzer: studies on the elimination of within-rotor drift.

We investigated the measurement of phenytoin in plasma by the EMIT method, in the Monarch centrifugal analyzer. When we used the standard protocol supplied by Instrumentation Laboratory, significant drift was observed across a full rotor loaded with 31 replicates of a single specimen. Although the cause remains obscure, the drift was eliminated by using a load-spin-reload-spin procedure. This makes it possible to analyze large batches of samples without excessive use of standards and reagents.

Studies on measurement of plasma magnesium: application of the Magon dye method to the "Monarch" centrifugal analyzer.

The "Magnesium Liquid Stable Reagent Set" from Medical Analysis Systems, Inc., is evaluated. The method, which involves Magon dye binding and bichromatic absorbance measurements, was used in a Monarch centrifugal analyzer. Results were compared with those by atomic absorption spectrometry. The calibration curve for the Magon method is linear to 2.5 mmol/L, with a 2-microL sample volume. Analytical recovery of magnesium added to human plasma ranged from 95 to 102%. The working reagent is stable for at least five days at 15 degrees C. At concentrations of 0.54 and 1.20 mmol/L, the respective CVs were 2.15 and 3.60% within batch, and 3.13 and 3.24% between batch. We analyzed 150 clinical samples for magnesium by both methods. Absorbance readings at 520/600 nm rather than 520/690 nm improved the correlation (r = 0.9777 and r = 0.9428, respectively). Calcium, albumin, phosphate, or bilirubin did not significantly interfere.

A multicentre european evaluation of the Kodak EKTACHEM GLU/BUN analyzer using NCCLS guidelines and other approaches.

This paper describes the field performance of the Kodak EKTACHEM GLU/BUN Analyzer for glucose and urea. NCCLS protocols PSEP-2, 3 an 4 were used which enable manufacturers to establish performance claims concerning the precision and accuracy of an analytical system. This multicentre trial used four analysers in four European countries, United Kingdom, France, West Germany and Italy to assess within and between laboratory performances. Freeze dried control materials were used for the performance check experiment PSEP-2 and for the replication experiment PSEP-3. The replication study, although very time consuming, was straight forward to undertake and the results were comparable between centres. Compliance with the protocol for comparison of methods experiment PSEP-4 in which laboratories used their own hospital patient samples was more difficult. The problems with obtaining suitable samples and the performance of comparative methods are discussed in detail.

Development of an enzyme immunoassay for human placental lactogen using labelled antibodies.

An immunoassay for the determination of human placental lactogen (HPL) has been developed using specific antibodies labelled with horseradish peroxidase. A method is presented in which the enzyme : antibody conjugate is purified with an antigenic solid phase to ensure the production of conjugates with intact immunological activity. Serum components interfere with the assay but this problem can only be avoided at the expense of a precise and practical assay for the determination of HPL.