CLIN.CHEM. 25/3.439-442(1979) AutomatedMeasurementof Total Cholesteroland Triglycerides,in “Tandem,” on the DiscreteSample Analyzer,GilfordSystem 3500 Lynn Smith, Diane Lucas, and Gary Lehnus We have developed an automated procedure on a discrete sample analyzer, Gilford#{174} System 3500, which alternately measures both total serum cholesterol and triglyceride concentrations as a “tandem” procedure. We used Dow Diagnostic’s fully enzymatic, colorimetric reagents and aqueous standards to calculate unknowns ratiometrically. Cholesteroland then triglyceride reagent aredispensed into alternatecups; the produced color at 500 nm is measured after an ambient temperature incubation of 20 mm. Reagent and sample carryover is less than 1.6%. Correlation coefficients of 0.997 for comparison of both automated tests with the manual methods at 30 #{176}C and a typical CV of less than 2.0% show this “tandem” procedure to be reliable and accurate. Cholesterol determinations have become they have been associated with various lipid disorders such as hyperlipidemia, diabetes mellitus, hypothyroidism, liver disease, atherosclerosis, and other metabolic derangements (1,2). The quantitation of both exceedingly Method and Materials Serum samples are loaded on the instrument in duplicate. Serum and either cholesterol or triglyceride reagent are separately dispensed into alternate cups. After a 20-mm incubation at ambient temperature, the reaction mixtures are sequentially aspirated and the amount of color formed is measured at 500 nm. The concentrations of total cholesterol or triglycerides in the samples are calculated ratiometrically, according to the respective standard’s absorbance. In the cholesterol reaction, samples are incubated with cholesterol esterase and cholesterol oxidase, which convert cholesterol esters to cholest-4-en-3-one and hydrogen peroxide. Peroxidase converts 4-aminoantipyrine, in the presence of hydrogen peroxide and phenol, to a quinone imine chromogen. and triglyceride important because lipid constituents is helpful in classifying familial hyperlipoproteinemias and, under appropriate circumstances, may be sufficient for diagnosis (3). Early clinical procedures required strong saponification chemicals and mineral acids in multistep processes (4-8). Modern procedures use microbial and animal enzymes to rapidly hydrolyze cholesterol and triglycerides, making them more adaptable to modern instrumentation (9-13). Although the Gilford System 3500 (Gilford Instrument Laboratories, Inc., Oberlin, OH 44074) is primarily a discrete sample analyzer, its programmable versatility allows it to be used as a dual-channel instrument. Prerequisites in the development of this tandem procedure include identical wavelengths, timing, temperature, and sample volumes; reagent compatibility; and the ability to under-reconstitute reagents for both tests. The tandem procedure uses test parameters similar to the reagent manufacturer’s manual methods. The differences between the two methods are the reduced sample and reagent volumes, incubation at ambient temperatures, and elimination of the addition of acid in the triglyceride test immediately after sample hydrolysis to stabilize the final color. The addition of the acid reagent is unnecessary in the tandem procedure because the tests are processed in an exact time se- cholesterol esterase Cholesterol esters cholesterol (EC 3.1.1.13) + fatty cholesterol oxidase Cholesterol + 02 cholest-4-en-3-one (EC 1.1.3.6) + H2O2 2H2O2 + 4-aminoantipyrine peroxidase + phenol quinone Inc., 132 lipases Triglycerides glycerol accepted glycerol Glycerol kinase + ATP glycerol (EC Glycerol 1-phosphate 1-phosphate + ADP 2.7.1.30) + NAB glycerol. 1-phosphate dehydrogenase (EC 1.1.1.8) Artino Street, Oberlin, Dec. 19, 1978. + fatty acids (EC 3.1.1:1) phosphate + NADH diaphorase NADH + INT 1978; chromogen The triglyceride reaction involves two microbial lipases that completely hydrolyze triglycerides to glycerol. Glycerol is converted to glycerol-1-phosphate by glycerol kinase, which then reacts with NAD in the presence of glycerol - 1-phosphate dehydrogenase to yield dihydroxyacetone phosphate and NADH. The reduced NADH in the presence of diaphorase reacts with INT [2- (p -iodophenyl) -3-p -nitrophenyl-5phenyltetrazolium chloride] to yield a colored formazan. dihydroxyacetone Laboratories, imine (EC 1.11.1.7) quence. Gilford Instrument OH 44074. Received July 26, acids formazan + NAD (EC 1.6.4.3) CLINICALCHEMISTRY,Vol. 25, No. 3, 1979 439 Table 1. Correlation of Tandem Cholesterol Determination with Two Other Methods Manual method Correlation coefficient Slope Intercept No. samples 0.997 1.002 0.03 55 Table 3. Within-Run Precision of Tandem Determination 3500 BMC procedure 0.993 1.014 -0.10 79 Table 2. Correlation of Tandem Triglyceride Determination with Two Other Methods Manual method Correlation coefficient Slope Intercept 0.997 0.958 0.03 No. samples 46 3500 BMC procedure 0.983 1.035 -0.19 67 Instrumentation The Gilford System 3500 discrete sample analyzer was used for the tandem automated procedure with program card and procedure (part no. 17831X82, Gilford). The reference instrument for the manual methods was a Model 250 Spectro(Gilford) with a temperature-controlled cuvet maintained at 25 ± 0.1 #{176}C. A water bath was used at 37 ± 0.5 #{176}C forincubation, and a Pipetter/Diluter (Gilford) was used to dispense samples. photometer Reagents Reagents supplied by Dow Diagnostics (Dow Chemical Co., P.O. Box 68511, Indianapolis, IN 46268) were used throughout. Each reagent was reconstituted to two-thirds regular volume to allow for the addition of water to dispense samples and to maintain final concentrations similar to those in the manual methods. Each vial of cholesterol reagent (cat. no. 46650) was reconstituted with 10.0 mL of the supplied buffer containing phenol, and the triglyceride reagent (cat. no. 46676) was reconstituted with 7.0 mL of the supplied tris(hydroxymethyl)methylamine buffer. The following reconstituted reagent concentrations (per liter) give concentrations similar to those used by the manufacturer when additional water is added: Cholesterol analysis. 15.5 mmol sodium cholate, 3.4 mmol 4-aminoantipyrine, 32 mmol phenol, 102 U cholesterol esterase, 186 U cholesterol oxidase, and 71 300 U peroxidase; buffer, pH 6.7. Triglycerides analysis. 79000 U lipases, 236 mg magnesium chloride, 1.4 g ATP, 5.7 g NAD, 190 mg INT, 44 000 U diaphorase, 720 U glycerol kinase, and 10 800 U glycerol-iphosphate dehydrogenase; buffer, pH 7.4. Reagents for the manual methods were prepared according to the package inserts. Cholesterol, TriglycerIdes, 20 20 20 Procedures Manual methods: Reconstitute the cholesterol 440 CLINICALCHEMISTRY,Vol.25,No.3,1979 reagent with g/L 20 3.230 0.763 1.555 2.608 3.865 0.030 0.006 0.007 0.037 0.029 0.93 0.76 0.44 1.41 0.76 Make calculations as described in the package inserts for both tests to obtain results in grams per liter. Automated procedure: Reconstitute cholesterol and triglyceride reagents with 10.0 mL and 7.0 mL of reconstituting buffer, respectively, and allow to equilibrate to ambient temperature for 10-15 mm. Program the instrument with the special program card. Dispenser A delivers 5 tL of sample and 0.25 mL of distilled water into each reaction cup. Dispenser B delivers 0.5 mL of cholesterol reagent into the first cup of each sample pair, and Dispenser C delivers 0.5 mL of triglyceride reagent into the second cup of each sample pair. In determining the cholesterol and triglyceride factors, the instrument divides the concentration of the standard by the difference between absorbance blank [standard conc/(Astendard absorbance values are converted of the standard - and of the factor]. Sample to grams per liter by the A blank) = following formula: in g/L. The blank (Asample Ablk) X factor = sample value and standard absorbances and the factors are printed as a check on the instrument/reagent system. Load the reaction strips over the sample cups and set the instrument controls as instructed by the tape printout; start the program by depressing the RUN push button. The samples are automatically processed in sequence, alternating cholesterol and triglyceride reagent addition. The instrument times the 20-mm hydrolysis period and aspirates the mixtures into the thermal cuvet, which is regulated at 25 ± 0.1 #{176}C. After a 5-s equilibration time, the instrument and prints the results. reads the absorbances Results Correlation samples, less than one week old and stored tightly available lyophilized controls were used to evaluate the tandem cholesterol! triglyceride procedure. Comparisons with manual methods and BMC’s System 3500 procedures (Tables 1 and 2) were made. The BMC triglyceride procedure uses a serum blank for each sample and calculates results from the difference in absorbance between blank and sample. The tandem cholesterol procedure correlates to the Liebermann-Burchard method (4,5) with a slope of 1.010 and a correlation coefficient of 0.98 1. The tandem triglyceride procedure correlates to the capped at 4 #{176}C until use, and commercially Aqueous cholesterol and triglyceride standards were provided by the manufacturer in the reagent kits. The cholesterol standard contained 2 g of cholesterol per liter in ethylene glycol monomethyl ether, and the triglyceride standard contained 2.08 g of glycerol per liter, equivalent to a triolein concentration of 2 g/L. 20 15.5 mL of buffer as described in the package insert and preincubate at 37 #{176}C for 7-45 mm. Pipet 10 L of blank, standard, and samples into disposable glass test tubes containing 1.5 mL of pre-warmed reagent, mix by inversion, and incubate at 37 #{176}C for 10 mm. Read absorbances at 500 nm within 15 mm after hydrolysis, to avoid color deterioration. Reconstitute the triglyceride manual method reagent as described with 5.5 mL of buffer solution and preincubate at 37 #{176}C for 5-10 mm. Pipet 20 sL of blank, standard, and samples into disposable glass test tubes containing 1.0 mL of pre-warmed reagent, mix by inversion, and incubate exactly 10 mm at 37 #{176}C; then immediately pipet 2.0 mL of acid reagent into each tube. Mix by inversion, then record absorbances at 500 nm. Patients’ Standards g/L No. 20 20 20 assays Mean 1.024 1.746 2.014 SD 0.008 0.028 0.014 CV, %0.83 1.63 0.67 Table 5. Results of Reagent Sample Carryover Tests Table 4. Day-to-Day Precision of Tandem Determination Cholesterol, g/L No. 24 assays 24 24 TrIglycerIdes, 24 24 24 % carryover g/L 24 24 Mean 1.453 2.080 2.314 1.995 0.830 1.460 1.817 3.079 SD 0.037 0.042 0.038 0.040 0.028 0.048 0.064 0.098 CV, %2.53 2.02 1.62 1.35 3.33 3.30 3.49 3.19 Worthington Diagnostic saponification method of 1.046 and a correlation coefficient of 0.994. with a slope . High Cholesterol to low triglycerides Low cholesterol to high triglycerides High triglycerides to low cholesterol Low triglycerides to high cholesterol 1.55 0.81 1.35 1.58 pies. Each set of four results was averaged to establish mean values for separate and tandem runs and used in a formula proposed by Broughton (14) in a scheme for instrument evaluation. Overall carryover was calculated to be less than Precision 1.6%(Table 5). Precision data are listed in Tables 3 and 4 for the tandem cholesterol/triglyceride procedure. Within-run precision was evaluated on replicate assays of four pooled sera having cholesterol values ranging from 1.02 to 3.23 g/L and triglyceride values ranging from 0.76 to 3.86 g/L. Results showed coeffi- Linearity cients of variation of less than 1.7% for cholesterol and 1.5% for triglycerides (Table 3). Day-to-day precision was accumulated over a three-day period on four pooled sera with concentrations ranging from 1.45 to 3.00 g/L for cholesterol and 0.83 to 3.08 g/L for triglycerides. Freshly reconstituted reagent and standards were used for each assay. Results showed coefficients of variation of less than 2.6% for cholesterol and 3.5% for triglycerides (Table 4). or both. Each pool was diluted with human sera with low amounts of analyte to give appropriate concentrations. Observed results, y, are plotted vs. expected results, x (Figures 1 and 2). Acceptable linearities were obtained up to 6 g/L for cholesterol and to 5 g/L for triglycerides. Values greater than these should be diluted with low-concentration sera and reassayed. Carryover Discussion Carryover studies were performed to determine the cross- contamination effects between the alternately aspirated cholesterol and triglyceride sample/reagent mixtures. Human samples were selected that contained abnormally high and low cholesterol and triglyceride concentrations. Each high or low sample was separately assayed in quadruplicate, then reassayed with the tandem procedure, alternating various con- centration combinations of cholesterol and triglyceride Linearities of the automated tandem procedure were determined by assaying dilutions of pools of human sera with abnormally high concentrations of cholesterol, triglyceride, Most discrete sample analyzers cannot be used as a dual- channel system to measure two serum constituents within the course of one analysis; the Gilford System 3500, however, can be used to routinely assay both total cholesterol and triglyceride serum concentrations as a tandem procedure. Assays for total cholesterol and triglyceride concentrations are frequent tests on patients exhibiting various lipid disor- sam- 7 N 6 5 C) C, a., 4.. 0) 0 a, > a) a) .0 4 C) > C, Cl) .0 3 0 0 2 1 Expected Fig. 1. Linear range of tandem (g/liter) cholesterol dilutions ofabnormalpatients’ samples procedure from Expected (g/liter) Fig. 2. Linear range of tandem triglyceride procedure from dilutions ofabnormalpatients’ samples CLINICAL CHEMISTRY, Vol. 25, No. 3, 1979 441 This unique dual-measuring procedure offers considerable savings over discrete determinations routinely used by ders. batch analyzers. Productivity of the tandem cholesterol/triglycerides procedure is 46 patient pairs/h. We have shown that this procedure is comparable to the manual methods, System 3500 BMC procedures, and to Lie- bermann-Burchard and saponification methods. It offers excellent precision reagents,minimum and repeatability with economical use of use of personnel, and readily available commercial reagents. Other reagents cannot be interchanged for this procedure. We thank Dr. Andris Indriksons and Robert Mack (Dow Diag- 5. Burchard, H., Beitrage zur Kenntnis des Cholesterins. Chem. Zentralbl. 61, 25 (1890). 6. Griguat, A., Proc#{233}d#{233} colorim#{233}trique de dosage de Ia cholest#{233}rine dans l’organisme. C. R. Soc. Biol. 68, 791 (1910). 7. Abell, L. L., Levy, B. B., Brodie, B. B., and Kendall, F. E., Simplified method for the estimation of total cholesterol in serum and demonstration of its specificity. J. Biol. Chem. 195, 357 (1952). 8. Wybenga, D. R., Pileggi, V. J., Dirstine, P. H., and DiGiorgio, J., Direct manual determination of serum total cholesterol with a single stable reagent. Clin. Chem. 12, 980 (1970). 9. Bucolo, G., and David, H., Quantitative determinations of serum triglycerides by the use of enzymes. Clin. Chem. 19, 476 (1973). 10. Flegg, H. M., Investigation of the determination of serum cholesterol by an enzymatic method. Ann. Clin. Biochem. 10, 79 (1973). nostics) for their help with this procedure. References 1. Fredrickson, D. S., Physician’s guide to hyperlipidemia. Mod. Concepts Cardiovasc. Dis. 41,31(1972). 2. Tietz, N. W., Fundamentals of Clinical Chemistry, W.B. Saunders Co., Philadelphia, PA, 1970, p 323, 329. 3. Davidsohn, I., and Henry, J. B., Clinical Diagnosis by Laboratory Methods, W.B, Saunders Co., Philadelphia, PA, 1974, p 626. 4. Liebermann, C., Ueberges Oxychinoterpen. Ben. Dtsch. Chem. Ges. 18, 1803 (1855). 442 CLINICALCHEMISTRY. Vol. 25,No.3, 1979 11. Richmond, W., Preparation and properties of a cholesterol oxidase from Nocardia sp. and its applications to the enzymatic assay to total cholesterol in serum. Clin. Chem. 19, 1350 (1973). 12 Allain, C. C., Poon, L. S., Chan, C. S. C., et al., Enzymatic determinations of total serum cholesterol. Clin. Chem. 20,470 (1974). 13. Roeschlau, P., Bernt, E., and Gruber, W., Enzymatic determination of total cholesterol in serum. J. Clin. Chem. Clin. Biochem. 12,226 (1974). 14. Broughton, P. M. G., Gowenlock, A. H., McCormack, J.J.,and Neill, D. W., Revised scheme for the evaluation of automatic instruments for use in clinical chemistry. Ann. Clin. Biochem. 11, 207 (1974).
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