Lidocaine metabolite and creatinine measurements in the Ektachem 700: steps to minimize its impact on patient care.

The single-slide method used for creatinine in the Kodak "Ektachem" analyzer is far more precise than the two-slide method. We confirm that sera from patients on intravenous therapy with lidocaine exhibit a positive bias in results for creatinine but that lidocaine itself does not interfere. Instead, N-ethylglycine, a metabolite of lidocaine with a structure similar to that of sarcosine, is probably the cause. A method that allows N-ethyglycine to be measured directly is described. We followed the degree of this interference through five generations of the slide. Our investigations include two detailed comparison studies between the Kodak Ektachem 700 and the Beckman Astra analyzers. Creatinine determinations on lidocaine-treated patients when first-generation slides were used averaged 4.6 mg/L higher than determinations on these same specimens performed in the Astra. Serum creatinine results from patients not on lidocaine showed no significant difference between the two instruments. The average difference in generation 2, 3, and 4 slides was 0.24, 0.22, and 2.5 mg/L, respectively. No more than 2% of our creatinine results had a clinically significant lidocaine-related bias. We show how to identify and correct this small proportion of results that are biased because of lidocaine therapy.

Measurement of ionized calcium in biological fluids: ion-selective electrode method.

There is now a consensus that Ca2+ measurements are more physiologically and clinically meaningful than CaT measurements. Ca2+ in serum, plasma, whole blood, and other biological fluids can be measured by direct potentiometry with ion-selective electrodes and a number of reliable instruments are commercially available for this measurement. Several factors affect the Ca2+ concentration and must be carefully controlled for the results to be meaningful. The most important of these considerations are the anaerobicity of the sample, the need to concurrently measure pH, and the concentration of heparin, if whole blood or plasma samples are used. The calibration of the Ca2+ ISE is critical to the accuracy of the measurement. The matrix of the calibrator should match that of the sample as closely as possible, particularly in regard to ionic strength and liquid junction potential. The measurement of Ca2+ in urine is complicated by the wide variation in ionic strength encountered in this type of sample; thus, it is more meaningful to standardize the ISE in terms of Ca2+ activity instead of concentration. Instrumentally, the measure of copper in biological samples can be achieved with high accuracy, high precision, without background correction, and with minimum sample pretreatment if care is taken to carefully plan and implement all the critical steps in the analysis procedure.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of ionized calcium in serum with ion-selective electrodes: a mature technology that can meet the daily service needs.

This article reviews key advances in ion-selective electrode technology that have made potentiometric measurements of ionized calcium (Ca2+) reliable and precise. Our use of two second-generation Ca2+ analyzers (Radiometer ICA1 and NOVA 8) made possible uninterrupted service as volume increased to 31 640 patient tests in 1985. The lower results on the NOVA 8 were adjusted upwards to match those of the ICA1 to give identical results. Both analyzers were evaluated under working conditions of high volumes and multiple operators to establish downtime, electrode life, and costs. We have classified all Ca2+ analyzers into first-, intermediate-, and second-generation instruments, the better to understand their differences. Results for large numbers of patients' sera were shown to be systematically different when any two analyzers were compared. These differences are the consequence of each manufacturer's unique choices of the following: (a) the matrix of the calcium calibration solutions, (b) the type and configuration of the reference electrode, and (c) the salt-bridge solution. Elimination of each analyzer's biases will require agreement on a reference system that defines the accuracy of Ca2+ measurements on serum, plasma, or whole blood. The sound analytical performance of today's second-generation Ca2+ analyzers has allowed us to exploit the inherent superiority of Ca2+ over total calcium (CaT) measurements in the daily care of patients. We report on the preference of Ca2+ over CaT by physicians at our hospital since the introduction of second-generation Ca2+ analyzers. Therefore, we state unequivocally from our very satisfactory experience over the past five years that Ca2+ is a clinical laboratory test whose time has come!