False negative GC/MS assay for carboxy THC due to ibuprofen interference.

A case of a false negative THC metabolite confirmation by GC/MS is presented. A urine specimen testing positive by the EMIT d.a.u. Cannabinoid 100-ng assay was subjected to confirmation by a GC/MS procedure designed to detect the methyl derivative of 11-nor-delta 9-tetrahydrocannabinol-9-carboxylic acid (THC-COOH). No methylated THC-COOH was found. Analysis of the specimen by the TOXI-LAB Cannabinoids TLC procedure yielded a positive confirmation. Subsequent work-up of the specimen revealed a high concentration of ibuprofen, which is shown to interfere with the methylation of THC-COOH. Reanalysis of the specimen by GC/MS with an increased amount of methylating reagent yielded a positive result.

Central effect of intravenous phenylephrine on baroreflex control of renal nerves.

Responses of renal sympathetic nerve activity were determined in eight chloralose-anesthetized rabbits during sustained (1-3 minutes) increases in arterial pressure induced by phenylephrine infusion, and as arterial pressure returned to control. In four of the eight experiments, aortic baroreceptor traffic was also recorded. When arterial pressure was raised from 81 +/- 5 to 110 +/- 7 mm Hg, renal nerve activity decreased from 30 +/- 7 to 1 +/- 1 imp/sec. Aortic nerve activity increased from 208 +/- 35 to 346 +/- 49 imp/sec. When pressure returned to control (81 +/- 5 mm Hg), renal nerve activity remained inhibited (7 +/- 2 imp/sec), even though aortic nerve activity had also returned to control (195 +/- 33 imp/sec). Arterial pressure and traffic in the renal and aortic nerves returned to control over the succeeding 1 to 5 minutes. Transient increases in arterial pressure (lasting less than 1 minute) due to bolus injections of phenylephrine resulted in inhibition of renal nerve traffic followed by rapid recovery. In five rabbits with aortic and vagal nerves sectioned and both carotid sinuses isolated from the circulation, intravenous phenylephrine infusion augmented the gain of the isolated carotid baroreflex (particularly at low carotid sinus pressures). In nine experiments, injection of phenylephrine (0.01, 0.1, or 1.0 microgram) into the lateral ventricles did not change the basal renal nerve traffic but augmented the gain of the baroreflex control of the renal nerves. Our data indicate that peripherally infused phenylephrine can alter the arterial baroreflex control of the renal nerves by a central effect. The similar influence of intracerebroventricular phenylephrine on baroreflex control of the renal nerves is consistent with this view.

Colorimetric method for free fatty acids in serum validated by comparison with gas chromatography.

We compared results by a simplified colorimetric copper-soap method for serum free fatty acids (Clin, Chem. 19: 419, 1973, modified) with those by a gas--liquid chromatographic method. The modified method requires only 100 microliters of sample, its standard curve is linear from 0.1 to 4.0 mmol/L (reference interval for adults: 0.2 to 0.8 mmol/L), and it is suitable for use with newborns. Comparison with the gas--liquid chromatographic method (n = 51) over a wide concentration range gave a correlation coefficient of 0.989. Between-run CVs varied from 4 to 10%, analytical recoveries from 97 to 104%. Triglycerides, salicylates, hemoglobin, and anticoagulants (except citrate) do not interfere, but bilirubin and phospholipids give small positive interferences. The colorimetric assay is used, along with other routine tests, to assess lipid status and estimate kernicterus risk in neonatal intensive-care patients receiving lipid nutrition parenterally.

Adaptation of the EMIT serum digoxin assay to a mini-disc centrifugal analyzer.

We have modified the EMIT (Syva) serum digoxin assay for use with a mini-disc centrifugal analyzer. A minicomputer is used to control data acquisition, calculations, and readout for 14 cuvettes simultaneously. Thus, it is not necessary for time each individual assay manually or to plot calibration curves on graph paper. We use incubation times and dilutions similar to those used for the manual procedure, but we have changed the buffer solution, decreased the measurment time, and decreased the required serum volume per assay fivefold. We evaluated day-to-day and within-run precision, using Syva calibrators and control sera. EMIT assay values for radioimmunoassay control sera correlate well with the supplier's stated values.

Automated enzymatic determination of L-lactate in serum, with use of a miniature centrifugal analyzer.

We describe an automated enzymatic reaction-rate method for spectrophotometric determination of lactate in serum with a miniature centrifugal analyzer. The L(+) -lactate is selectively oxidized in the presence of lactate dehydrogenase (EC 1.1.1.27) and NAD+ to form NADH, which is measured from its absorption. Reaction rates are determined automatically, and unknown concentrations are calculated from a computer-generated calibration curve with aqueous lithium lactate standards. Lactate concentrations in the range 0.32-1.6 mug/4 mul (80-400 mg/liter) of sample were determined with relative errors and coefficient of variation of 4.8%. Analytical recovery of lactate added to pooled serum was 89-112% (average, 101%). Comparison with a kit ("Rapid Lactate") method gave a correlation coefficient squared of 0.979 over a concentration range of 39-779 mg/liter.

Adaptation of "EMIT" technique for serum phenobarbital and diphenylhydantoin assays to the miniature centrifugal analyzer.

We used a miniature centrifugal analyzer in a spectrophotometric rate-measurement mode to determine the anticonvulsant drugs phenobarbital and diphenylhydantoin in serum, by use of a modified enzyme immunoassay ("EMIT", Syva Corp.) We decreased reagent cost per determination by at least sixfold by means of microscale techniques. Also, the analysis rate is increased by measuring multiple samples simultaneously. Our method requires only 3 mul of serum for duplicate determinations. Replicate analyses of sera containing phenobarbital and diphenylhydantoin gave reaction rates with a CV of 1.5%. Run-to-run CV was 15%. Analytical recovery for drug-supplemented serum samples was 98%, and results for a series of samples compared well with results obtained by gas chromatography (for phenobarbital, r = 0.95; for diphenylhydantoin, r = 0.91).