Sulfapyridine has no effect on the measurement of serum iron by coulom- etry. Moreover, drugs with structural similarities to sulfasalazine, phenazo- pyridine.
Observations on Comparisons of Within-Run and Day-to-Day Precision
day-to-day imprecision to mean 5T or s. This could be prevented if ST were called total imprecision and SD day-to-day component of imprecision (4). Finally, the NCCLS-proposed standard (2) suggests that for data collected as a series of runs of n replicates over k days, the grand standard deviation (‘.JSST/(kn 1), from the analysis of variance table) gives an approximation of 5T One should be aware, however, that this approximation is an underestimate of 5T = \/SW2 + SD2 and the underestimate may be appreciable unless n/k is small (5). Thus,
To the Editor: In Clinical Chemistry, one occasionally encounters precision data wherein day-to-day imprecision is reported as less than within-run imprecision. One also finds comparisons of day-to-day and within-run imprecision made at slightly different analyte concentrations. This raises the question of what is meant by day-to-day imprecision and how it is calculated. The standard analysis of variance method of estimating imprecision from a series of runs of n replicates over a period of k days can be summarized by the following analysis of variance table and the equations after it (1-3). To simplify matters, a between-run within-day component has been omitted (i.e., one run per day). Analysis
Source
Sums
Day-to-
SSD
if w2 and so2 then ST2
k
=
=
{[SSD/(k - 1)] - sw2Vn (SSD + SSW)/(kn 1) I(nso2 + sw2)(k - 1)
=
+ sw2k(n
of variance
1)]
1)]/(kn
-
-
-
1)
1)I/(kn
-
table
Mean MSD
-
Fsw2+ sp2lln(k
=
1
-
SSW/Ik(n
=
Degrees of freedom
of squares
=
=
Quantity estimated by mean square
square SSD/(h
-
1)
w2
+ n(x1)-
k
day Within-
n>J(x1-x)2 SW k
n
k(n
=
1)
-
MSW
=
SSW/(k(n
-
1)]
run Total
kn
-
1
(x1-i12 i=1
j=I
Estimates (within-run) = V’MSW 5D (day-to-day_component) ST (total) = \75W2 + s2
=
v’(MSD
Several conclusions are readily apparent: 1. Calculated by the analysis of variance method, there is only one mean for within-run, day-to-day component, or total imprecision, because all of the data are used to calculate each statistic. 2. Although within-run imprecision could be calculated from a single run, in the analysis of variance method, use of all of the data provides a more reliable estimate, not only because of the greater degrees of freedom, but also because environmental factors that affect precision and that can change daily have been pooled. 3. If day-to-day imprecision implies total imprecision, ‘r, as would he assumed by many readers, then this value cannot be less than the within-run imprecision. This is both intuitive and a consequence of the equation for s’-. On the other hand the day-to-day component, 5D can be less than the within-run imprecision. One source of confusion is whether investigators and readers understand 202
CLINICAL CHEMISTRY,
of imprecision
_______________
w
-
MSW)/n
E. K., Kanofsky,
P., Shakurji,
G.,
and Cutlove, E., Biological and analytic components of variation in long-term studies of serum constituents in normal subjects. II. Estimating biological components of variation. Clin. (‘hem. 16, 1022-1027 (1970). 2. NCCLS Proposed Standard: PSEP-3 Protocol for Establishing l’erformance Claims for Clinical Chemical Methods. Replication Experiment. NCCLS, 771 E. Lancaster Ave., Villanova, PA 19085, 1979. 3. Davies, 0. L., The Design and Analysis of Industrial Experiments, 2nd ed., Longman, New York, NY, 1978, pp 102-104. 4. Rodbard, D., Statistical quality control and routine data processing fQr radioimmunoassays and immunoradiometric assays. Clin. (‘hem. 20,1255-1270(1974). 5. Brownlee, K. A., Statisticat Theory and Methodology in Science and Engineering, 2nd ed., John Wiley and Sons, New York, NY 1965, p 323.
Jan S. Krouwer Technicon Instruments 511 Benedict Ave. Tarrytown,
Vol. 27, No. 1. 1981
To the Editor:
Recently ESA (Environmental Sciences Associates, Inc., Bedford, MA 01730) marketed an instrument, the Ferrochem 3050, designed to measure serum iron by use of coulometry. Published results (1 ) indicate this method to he more rapid and to require less serum than other methods and to be unaffected by drugs in serum. Sulfasalazine (salicylazosulfapyridine, Azulfidine#{174};Pharmacia), used in the treatment of Crohn’s disease, interferes with this measurement of serum iron. Serum iron concentrations of two Crohn’s disease patients were 5.8 and 7.0 1g/L when determined electrothermally by graphite furnace atomic absorption; the apparent values as measured with 3050 were 82.6 and 17.9 1) the Ferrochem pg/L, respectively. Both patients were receiving 2 g of sulfasalazine per day. The intestinal absorption of sulfasalazine has been extensively investigated (2-4). It is cleaved by bacteria in the colon to 5-aminosalicylic acid and sulfapyridine, the latter compound being extensively metabolized after absorption (4, 5). Although sulfapyridine has been demonstrated to be the compound that is predominantly absorbed, up to 30% of the parent drug can be absorbed. The metabolism of sulfasalazine does not result in any detectable compounds with an intact azo linkage (2). Ulcerative colitis patients who are receiving between 3 and 6 g of sulfasalazine per day have been shown to have drug concentrations in their serum of 10-15 mg/L (6).
References 1. Harris,
Sulfasalazine Interferes with Coulometry of Serum Iron
NY
10591
Corp.
Sulfasalazine added to pooled serum increases the apparent concentration of serum iron, as measured with the Ferrochem 3050, mimicking the effect seen when the drug was administered orally. In contrast, addition of salfasalazine did not alter the value as determined electrothermally. An in vitro drug concentration of 13.2 mg/L was equivalent to an apparent serum iron concentration of 86.4 mg/L. When drug concentration is plotted vs the apparent serum iron value, the slope of the regression line is 1.8 and the linear correlation coefficient is 0.993. Sulfapyridine has no effect on the measurement of serum iron by coulometry. Moreover, drugs with structural similarities to sulfasalazine, phenazopyridine hydrochloride and azosulfamide,’ tested in vitro in concentrations up to 12.0 mg/L, did not alter the results of Ferrochem 3050 determinations. Sulfasalazine is used to treat ulcerative colitis and Crohn’s disease (5, 7). A I
Kindly supplied by 1)r. T. V. ChristoferIndustries, Elwood, KS
son (Medico (i13)24).
