concentrations of approximately 1 g/L of urine, which are commonly achieved 2 to 3 h after intravenous infusion, a false-positive result for Bradshaw’s test will occur. We wish to alert clinical biochemists and physicians to this source of interference, and stress the necessity of confinning. the validity of all screening tests for Bence Jones protein by measuring total urinary protein and using specific electrophoretic and immunochemical tests to detect light chains in fresh concentrated urine. References 1. Johnson phoresis 1797-1800 2. AM. Immunofixation electroClin Chem 28, (1982). and electrofocusing. Gale DSJ, Versey JMB, Hobbs JR. Rock- et immunoselection for detection of heavychain diseases. Clin Chem 20, 1292-1294 (1974). F.R.G.), for which the detection limit is (2). In no case did we detect immunoreactive material in undiluted specimens. The absorbances at 492 urn ranged from 0.020 to 0.040, no different from that of blank incubations (0.020 0.6 g/L to 0.035). Similar negative were also obtained fluids results for cerebrospinal concentrated fourfold would surely brane filtration. We conclude that, in contradiction to the previous report (3), immunoreactive PSG1 is either absent from cerebrospinal fluid or is present in concentrations definitely below 0.15 g/L. Because the sensitivity of the enzyme immunoassay (0.6 g/L) is at least equal to or greater than that reported for the radioimmunoassay (1 g/L), insensitivity of the applied method cannot be the reason for our failure to confirm the presence of immunoreactive PSG1 in human cerebro- be reflected ciency testing. The introduction handling by mem- exhibit typical of inter-lab s’ra. sodium results in the area of 1.5 mmol. A typical drift of 10 to 20 mniollL in such profi- into the sample- system ofsodium, potassium, chloride, and calcium for an instrument designed to analyze these constituents may be somewhat risky. Sodium drift of the magnitude reported is not normal, and it is suggested that the instrument in question be given a cbser look. Illegal grounding, salt bridges, and static charges are possible offenders in a situation like this. Thomas M. Happe ASTRA Applications Beckman Instruments, Brea, CA 92621 Dept. Inc. Urine Urea Nitrogen Astra-8 as Measured in spinal fluid. 3. Bradshaw TR. The recognition pathic albumin in urine. Br Med of myeloJ ii, 1442- 1444 (1960). 4. Hobbs JR. Bence-Jones proteins. Essays MedBiochem 1, 105-131 (1975). Peter Dennis Beryl Biegler Ian Larmour’ Depts. ofChem. Pathol. and ‘Pharmacy Prince Henry’s Hospital Melbourne, 3004, Australia References 1. Bohn H, Schmidtberger R, Zilg H. Isolierung des schwangerschaftsspezifischen f31-Glykoproteins (SP1) und antigenverProteine wandter durch Immunadsorption. Blut 32, 103-113 (1976). 2. Dati F, Bauer 11W,Schuster E, Bartos the Beckman S. Die diagnostische Relevanz des schwangerschafts-spezifischen /31-Glykoproteins bei verschiedenen malignen Erkrankungen. J Clin Chem Clin Biochem 20, 651-652 (1982). 3. Heikinheimo M, Wahlstrom T, L.ehto VP, et al. Pregnancy-specific $1-glycoprotein-like material in human cerebrospinal fluid. J Clin Endocrinol Metab 55, 189-192 Questioning the Presence of Pregnancy-Specific$1-Glycoprotein (or Immunologically Similar Material) in Cerebrospinal Fluid (1982). Axe! M. Gressner Dept. ofClin. Chem. RWTH Klinikum Aachen, F.R.G. Pregnancy-specific /31.glycoprotein (PSG1), a high-molecular-mass glycoprotein that normally is synthesized by the syncytiotrophoblasts during pregnancy, is increased in serum not only in pregnancy but also in patients with trophoblastic and, less frequently, with non-trophoblastic malignancies (1, 2). It is absent (<0.3 g/L) from the sera of non-pregnant, healthy female and male subjects (2), but recently the presence of PSG1 in cerebrospinal fluids of men and women was reported (2). The range of concentrations, as determined by radioimmunoassay (detection limit 1 gfL), was 1.9 to 3.6 pg/L (3). A correlation with other factors or with any specific cerebral diseases could not be established. We studied PSG1 in 15 cerebrospinal fluid specimens, from children and adult patients with cerebral tumors and inflammatory and ischemic cerebral diseases. We used a solid-phase enzyme-linked immunoassay (Enzygnost-SP1;Behringwerke AG, Marburg, Sodium Drift ASTRA: To the Editor: The Letter of Seddon et al. (Clin. 29: 212, 1983) deserves cornment in the interest of all riu users. We agree with the authors that an “upward drift of 10 to 20 rnmol/L in results for sodium” is not acceptable. We disagree, however, that an ionic wash solution provides an adequate fix for the problem. The fact that a sodium hypochionte rinse (not a recommended procedure) will “temporarily solve this drift” suggests some type of containinaChem. problem before assay. diluting In the Beckman in the sample-handling system of the particular In the Beckman Astra discrete analyzer, ion-selective electrode technology is used. The urea nitrogen method in this system was designed for use at conductivities normally found for planma. When plasma samples contain high, out-of-range concentrations of urea nitrogen, the samples routinely are diluted with physiological saline Because A Response tion To the Editor: the salt concentration of widely, the question of urines with either physiologi- urea varies - To the Editor: 1310 Chemistry Survey, standard deviations instrument cal saline or water before assay for urea nitrogen becomes an important issue with the Astra. Blumenfeld and Griffith (1) exammed extensively the Astra’s performance with urine Nat, K, C1, urea nitrogen, glucose, and creatinine, but they did not comment on the preferred diluent to use with a particular categoly of assay. We have compared urine urea nitrogen as measured with the Astra-8 and the American Monitor KDA over a wide range of concentrations with isotonic saline as the diluent. In the KDA, the enzymatic urease procedure is used for urea nitrogen analyses. Thirty patients’ samples, analyzed by both methods, gave mean values that cornpared favorably (KDA = 87.39 nunol/L vs Astra = 83.92 mrnolIL). In our laboratory, we routinely di- lute urine urea nitrogen samples with involved. physiological proficiency-survey programs, such as the College of Amencan Pathologists’ Comprehensive inter-patient variability in urinary salt concentrations. With this approach, we have not experienced the analytical Ongoing CLINICAL CHEMISTRY, Vol. 29, No. 6, 1983 saline to minimize the fluctuations in urine urea nitrogn re- ported by other Astra users with this method. Reference 1. Blumenfeld TA, Griffith B. Measurement of urinary constituents with the As- tra-8 multi-channel analyzer. Clin Chem 26, 1883-1886 (1980). cipitant), successively decreasing volnines of serum were added at room temperature. After their contents were by the difficulty mixed, the tubes stood for 15 mm before being centrifuged at 4 #{176}C for 30 mm. The cholesterol in each supernate was measured with use ofan ABA-100 discrete analyzer and Bio-Dynamics/ cated to the kit manufacturer, ElectroNucleonics, Inc. Their response pointed out that the package insert in the kit makes no claims about the total choles- cholesterol reagents. The lipoprotein content of each supernate was monitored by agarose gel electrophoresis (5) to identify the ed out the recognized facts that the results of HDL-C assays depend on bmc Laurence M. Demers Brenda Dourte Barbara Hutchinson Dept. of Pat/wi. The Milton S. Hershey Med. Center The Pennsylvania State Univ. enzymatic true” Tube no. Evaluation of a Kit to Measure HDL (HDL-C) in Serum To the Editor: A clinic laboratory got low values for serum HDL-C on using a kit supplied by Electro-Nucleomcs, Inc.’ In the kit, the precipitating reagents (phosphotungstate and MgSO4) are supplied as solids in a tube, to which 0.5 mL of serum is added according to the manufacturer’s package insert. We analyzed split human serum samples in parallel with such kits and using our in-house phosphotungetate method, a procedure like that of Lopes- Virella the pH gent to only 80 et al. (1), except that we adjust of the phosphotungstate 7.4 or slightly rea- less (2), and add plus 20 LL of MgCl2 reagents to 1.0 mL of serum. Values obtained with the kit were from 30 to 170 mg/L lower than pL ofphosphotungstate results with the in-house method, differences averaging 18.2% (SD 5.4%, n = 16) for values ranging from 280 to 1120 mg/L. Such differences would cause many samples with total cholesterol/HDL-C ratios near 5.3, based on the in-house method, to have ratios near 6.5 with the kit. These higher ratios would imply an importantly greater risk of coronary artery value (the cholesterol in the supernate disease (3). We further evaluated the kit by titrating (4) three human serum sam- ples with increasing ratios of precipitent to serum. To a series of kit tubes (each contained the prepackaged pre- Serum vol, mL Apparent mg/L HDL-C, 1 2 3 4 1.8 1.5 1.3 1.2 470 5 1.1 400 6 1.0 390 7 8 0.9 0.8 380 380 9 10 0.7 0.5 380 350 480 390 380 1 Use of products from named companies does not imply that the products are superior to similar products of comparable grade from other companies, nor does it constitute endorsement of the products by the United Force. such a terolJHDL-C ratio. Further, they pointconditions and reagents and that there were differences in both of these when our phosphotungstate reagents and method were compared and the instructions with in the their kit package insert. References 1. Lopes-Virella MF, Stone P, Ellis S, Colwell JA. Cholesterol determination in highdensity lipoproteins separated by three different methods. Clin Chem 23, 882-884 (1977). 2. Grove TH. Effect of reagent pH on deterrnination ofhigh-density lipoprotein cholesterol by precipitation with sodium phosphotungstate. Clin Chem 25, 560-564 (1979). 3. Uhl GS, Troxler RG, Hickman JR, Clark D. Relation between high density lipoprotein cholesterol and coronary artery disease in asymptomatic men. Am J Cardiol 48, 903-910 (1981). Because agarose gel electrophoresis showed no LDL or VLDL in the supernates of tubes 3-10, the “true” HDL-C concentration appears to be 390 mg/L (tubes 3-6). The HDL-C concentration measured by our in-house phosphottmgstate method was 390 mgIL, while the concentration measured with 0.5 mL of serum added to the kit tube was 350 mg/L. Similarly, in the other two titrations also, the HDL-C values were low when 0.5 mL of serum was added to the kit tube. With 1.0 mL per tube, however, agreement was satisfactory. The clinic laboratory then split a group of serum samples, measured the HDL-C in one aliquot with use of 1.0 mL of serum per kit tube, and sent us the other aliquot for analysis by our inhouse method. All values measured at the clinic with the kit were 10 to 40 mg/L lower than our values, which. ranged from 260 to 630 mg/L. The differences between the methods averaged 5.8% (SD 3.2%, n = 8). Although these differences were statistically significant, they were deemed acceptable because they were small and ent. Compared with the earlier States Air from which all VLDL and HDL had been removed by precipitation without detectable loss of HDL). Results from one serum were as follows: Hershey, PA 17033 Cholesterol HDL-C concentration in measuring volume. The above findings were communi- consist- differ- ences when 0.5 mL per tube was used, the improvement is obvious. Data from the other titrations suggest that a serum volume slightly less than 1 .0 mL (e.g., 0.85 mL) would be optimal, but the gain in accuracy is apt to be offset 4. Clark DA, Wood JA. Titration of serum lipoproteins with lipoprotein precipitants. USAF School of Aerospace Med., Technical Report SAM-TR-81-31, Brooks Air Force Base, TX 78235, 1981. 5. Noble RP, Hatch VF, Mazrimas JA, et al. Comparison of lipoprotein analysis by agarose gel and paper electrophoresis with analytical ultracentrifugation. Lipini 4, 55-59 (1969). Dale A. Clark R. Rozell E. L. Mosser Phillip Clin. Sci. Div. USAY School ofAerospace Med. Brooks Air Force Base, TX 78235 Cefoxitin Interference with Urinary 17-Hydroxycorticosteroid Determination To the Editor: Because of the marked minute-tominute variation in plasma cortisol concentrations, urine or total either free cortisol in 17-hydroxycorticoster- oids (17-OHCS) excretion is the best index to steroid output. The normal response to dexamethasone has been better established for urinary 17OHCS than for free cortisol (1); thus, accurate determination of urinary 17OHCS is essential. With the large increase in the num- ber ofpharmacological agents in recent CLINICAL CHEMISTRY, Vol. 29, No. 6, 1983 1311