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WITHIN-FAMILY BIRTH-WEIGHT STANDARDS
820 in the Mediterranean type of G.-6-P.D. deficiency.2 In our series the high prevalence (25-8%) of acute renal failurea complication rarely reported in G.-6-P.D.deficient patients3 and, it seems, extremely uncommon nowadays in children (2 cases of anuria in over 500 cases of favism 4)-is at least partly attributable, in our opinion, to the existence of a renal unit in our hospital (a third of the patients having been admitted as " anuria of obscure
PERCENTAGE OF
10TH
SECOND, THIRD, AND FOURTH BABIES BELOW THE. 90TH PERCENTILE OF TANNER’S WITHIN-
OR ABOVE THE
FAMILY
STANDARD
PRECEDING
ACCORDING
TO
MEAN
BIRTH-WEIGHT
OF
SIBS*
origin "). The severity of the illness (3 deaths) is also exceptional.5 Delay in referral to hospital and bad clinical condition on admission, high incidence in aged patients (50% over fifty 6 years old), and perhaps genetic factors may be implicated. Renal Unit, 2nd Department of Internal Medicine, University of Athens, Hippocration Hospital, Athens, Greece.
A. SYMVOULIDIS S. VOUDICLARIS TH. MOUNTOKALAKIS H. POUGOUNIAS.
WITHIN-FAMILY BIRTH-WEIGHT STANDARDS
SiR,—The within-family birth-weight standards pubcolleagues (July 29, p. 193) appear to provide an interesting new tool for research-workers. Unfortunately the practical application of the new standards will yield misleading and uninterlished by Professor Tanner and his
pretable results. I find the concept of comparing a newborn baby to his sibs, rather than to a population standard, very attractive. I am also in complete agreement with the authors about the degree of birth-weight similarity within sibships, with their estimates of within-sibship variation and of the distributions of birth-weight differences between a given sib and the The way in which these mean weight of preceding sibs. of are information put together in order to arrive at pieces birth-weight standards is, however, in error. The system presupposes that each mother has a charac" true " birth-weight to which a newborn baby can be compared. If this could be done no difficulty would arise. Since the " true " birth-weight is not known, the authors use the mean birth-weight of the preceding sibs. Why this must lead to serious misclassification (except for very large sibships) is best explained by an example. " Suppose we have two mothers, each having a true characteristic of 3000 No. g. 1 gives birth to birth-weight a first baby of 3400 g. and no. 2 to a first baby of 2600 g. Both weights are easily within the bounds of chance, even in the absence of disease, since the standard deviation around the " true " mean is 300 g. Following the prescribed within-family standard procedure, the actual weights of the first babies (after suitable corrections) are used to categorise second babies. That of mother no. 1 has a negligible chance of falling above the 97th centile but about a 10% chance of falling below the 3rd centile; whereas the reverse is true of the second baby of mother no. 2. In general, whenever the mean birth-weight of previous babies is appreciably higher than the true characteristic of the mother, the chances of classifying a subsequent baby as of low birth-weight are increased, and vice versa. Of course, the larger the sibship the better is the estimate of the characteristic birth-weight and the smaller the chance of misclassification, but the table shows that the bias is quite large for up to four sibs. The peculiarity of the within-family standards shown would not be looked for and would be unlikely to be noticed when the new standards
teristic
2. ibid.
1969, ii,
1177.
3. Whelton, A., Donadio, J. V.,
Jr., Elisberg, B.
L. Ann. intern. Med.
1968, 69, 323. 4. Kattamis, C. Personal communication. 5. Campanacci, L., Naccarato, R. Panminerva med. 1966, 8, 217. 6. Kattamis, C., Haidas, S., Metaxotou-Mavromati, A., Matsaniotis, N. Br. med. J. 1972, iii, 470.
Data from Aberdeen cohort study.l All birth-weights for length of gestation, sex, and parity.
are
corrected
used to select small contrasting groups of cases for further study. There is, therefore, a real danger of selecting for study groups of cases which are seemingly homogeneous (say, under the 3rd centile) but in reality represent correctly and incorrectly classified cases mixed in unknown proportions. Contrasts between so selected groups may hide real or reveal non-existent differences, and the truth would be almost impossible to determine retrospectively. are
M.R.C. Reproduction and Growth Unit, Princess Mary Maternity Hospital, Great North Road, Newcastle NE2 3BD.
W. Z. BILLEWICZ.
ALCOHOLIC HYPERLIPIDÆMIA
SIR,-Dr. Chait and his colleagues (July 8, p. 62) conclude from a metabolic study of patients with alcoholic hyperlipidxmia that hypertriglyceridxmia generally improves after withdrawing alcohol, and that the removalrate of exogenously administered triglycerides does not change significantly after withdrawal. We have tested the same variables in a series of 40 patients consecutively admitted to an alcoholic ward. Blood was drawn 12-14 hours after admission in fasting subjects who had not received any pharmacological treatment. Cholesterol and triglycerides were assayed semiautomatically,2,3 and free fatty acids (F.F.A.) by the coppernitrate method.44 Lipoprotein electrophoresis on agarose gel5 was carried out on each sample, and, in selected cases, preparative ultracentrifugation according to Havel et al. 6 was performed. No subjects with floating &bgr;-lipoprotein were detected. The findings in a series of normal subjects tested during the same period of time and in the alcoholics are shown in the accompanying table. The alcoholic subjects were divided into three groups: (a) chronic alcoholics; (b) acute alcoholics (i.e., subjects with acute signs of intoxication); and (c) patients with delirium tremens (D.T.). Considering that the upper limits of normal (mean±2
s.D.) in our control population were respectively 170 for triglycerides and 266 for cholesterol, we divided our Billewicz, W. Z., et al. Br. J. prev. soc. Med. 1970, 24, 97. Block, W. D., Jarrett, K. J., Levine, J. B. in Automation in Analytical Chemistry (edited by L. T. Skeggs, Jr.); p. 345. New York, 1965. 3. McLellan, G. H. Clin. Chem. 1971, 17, 535. 4. Duncombe, W. G. Clinica chim. Acta, 1964, 9, 122. 5. Sirtori, C. R., Hassanein, K., Hassanein, R., Boulos, B. M. ibid. 1971, 31, 305. 6. Havel, R. J., Eder, H. A., Bragdon, H. J. J. clin. Invest. 1955, 35, 1345.
1. 2.
PERCENTAGE OF
10TH
SECOND, THIRD, AND FOURTH BABIES BELOW THE. 90TH PERCENTILE OF TANNER’S WITHIN-
OR ABOVE THE
FAMILY
STANDARD
PRECEDING
ACCORDING
TO
MEAN
BIRTH-WEIGHT
OF
SIBS*
origin "). The severity of the illness (3 deaths) is also exceptional.5 Delay in referral to hospital and bad clinical condition on admission, high incidence in aged patients (50% over fifty 6 years old), and perhaps genetic factors may be implicated. Renal Unit, 2nd Department of Internal Medicine, University of Athens, Hippocration Hospital, Athens, Greece.
A. SYMVOULIDIS S. VOUDICLARIS TH. MOUNTOKALAKIS H. POUGOUNIAS.
WITHIN-FAMILY BIRTH-WEIGHT STANDARDS
SiR,—The within-family birth-weight standards pubcolleagues (July 29, p. 193) appear to provide an interesting new tool for research-workers. Unfortunately the practical application of the new standards will yield misleading and uninterlished by Professor Tanner and his
pretable results. I find the concept of comparing a newborn baby to his sibs, rather than to a population standard, very attractive. I am also in complete agreement with the authors about the degree of birth-weight similarity within sibships, with their estimates of within-sibship variation and of the distributions of birth-weight differences between a given sib and the The way in which these mean weight of preceding sibs. of are information put together in order to arrive at pieces birth-weight standards is, however, in error. The system presupposes that each mother has a charac" true " birth-weight to which a newborn baby can be compared. If this could be done no difficulty would arise. Since the " true " birth-weight is not known, the authors use the mean birth-weight of the preceding sibs. Why this must lead to serious misclassification (except for very large sibships) is best explained by an example. " Suppose we have two mothers, each having a true characteristic of 3000 No. g. 1 gives birth to birth-weight a first baby of 3400 g. and no. 2 to a first baby of 2600 g. Both weights are easily within the bounds of chance, even in the absence of disease, since the standard deviation around the " true " mean is 300 g. Following the prescribed within-family standard procedure, the actual weights of the first babies (after suitable corrections) are used to categorise second babies. That of mother no. 1 has a negligible chance of falling above the 97th centile but about a 10% chance of falling below the 3rd centile; whereas the reverse is true of the second baby of mother no. 2. In general, whenever the mean birth-weight of previous babies is appreciably higher than the true characteristic of the mother, the chances of classifying a subsequent baby as of low birth-weight are increased, and vice versa. Of course, the larger the sibship the better is the estimate of the characteristic birth-weight and the smaller the chance of misclassification, but the table shows that the bias is quite large for up to four sibs. The peculiarity of the within-family standards shown would not be looked for and would be unlikely to be noticed when the new standards
teristic
2. ibid.
1969, ii,
1177.
3. Whelton, A., Donadio, J. V.,
Jr., Elisberg, B.
L. Ann. intern. Med.
1968, 69, 323. 4. Kattamis, C. Personal communication. 5. Campanacci, L., Naccarato, R. Panminerva med. 1966, 8, 217. 6. Kattamis, C., Haidas, S., Metaxotou-Mavromati, A., Matsaniotis, N. Br. med. J. 1972, iii, 470.
Data from Aberdeen cohort study.l All birth-weights for length of gestation, sex, and parity.
are
corrected
used to select small contrasting groups of cases for further study. There is, therefore, a real danger of selecting for study groups of cases which are seemingly homogeneous (say, under the 3rd centile) but in reality represent correctly and incorrectly classified cases mixed in unknown proportions. Contrasts between so selected groups may hide real or reveal non-existent differences, and the truth would be almost impossible to determine retrospectively. are
M.R.C. Reproduction and Growth Unit, Princess Mary Maternity Hospital, Great North Road, Newcastle NE2 3BD.
W. Z. BILLEWICZ.
ALCOHOLIC HYPERLIPIDÆMIA
SIR,-Dr. Chait and his colleagues (July 8, p. 62) conclude from a metabolic study of patients with alcoholic hyperlipidxmia that hypertriglyceridxmia generally improves after withdrawing alcohol, and that the removalrate of exogenously administered triglycerides does not change significantly after withdrawal. We have tested the same variables in a series of 40 patients consecutively admitted to an alcoholic ward. Blood was drawn 12-14 hours after admission in fasting subjects who had not received any pharmacological treatment. Cholesterol and triglycerides were assayed semiautomatically,2,3 and free fatty acids (F.F.A.) by the coppernitrate method.44 Lipoprotein electrophoresis on agarose gel5 was carried out on each sample, and, in selected cases, preparative ultracentrifugation according to Havel et al. 6 was performed. No subjects with floating &bgr;-lipoprotein were detected. The findings in a series of normal subjects tested during the same period of time and in the alcoholics are shown in the accompanying table. The alcoholic subjects were divided into three groups: (a) chronic alcoholics; (b) acute alcoholics (i.e., subjects with acute signs of intoxication); and (c) patients with delirium tremens (D.T.). Considering that the upper limits of normal (mean±2
s.D.) in our control population were respectively 170 for triglycerides and 266 for cholesterol, we divided our Billewicz, W. Z., et al. Br. J. prev. soc. Med. 1970, 24, 97. Block, W. D., Jarrett, K. J., Levine, J. B. in Automation in Analytical Chemistry (edited by L. T. Skeggs, Jr.); p. 345. New York, 1965. 3. McLellan, G. H. Clin. Chem. 1971, 17, 535. 4. Duncombe, W. G. Clinica chim. Acta, 1964, 9, 122. 5. Sirtori, C. R., Hassanein, K., Hassanein, R., Boulos, B. M. ibid. 1971, 31, 305. 6. Havel, R. J., Eder, H. A., Bragdon, H. J. J. clin. Invest. 1955, 35, 1345.
1. 2.