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Pediatric Endocrinology |
Association France Hypophyse (J.-C.C., J.-L.C., Y.L., J.-C.J.) and Institut de Recherche Thérapeutique (J.-P.T., M.L.) and Departement de Biostatistique et dInformatique Médicale (J.C.), Hôpital Cochin, Paris, France
Address all correspondence and requests for reprints to: Dr. Jean-Claude Carel, INSERM U-342 and Department of Pediatric Endocrinology, Hôpital Saint Vincent de Paul, 82 avenue Denfert Rochereau, 75014 Paris, France. E-mail: jccarel{at}infobiogen.fr
| Abstract |
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| Introduction |
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The organization for GH distribution in France allowed the centralized collection of relevant parameters for all patients treated in the country since the early 1970s. We have analyzed the results of all GHST that led to the initiation of treatment in 3233 children from 1973 to December 1989.
| Subjects and Methods |
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The following data were collected: date of initiation of GH treatment, height, weight, chronological age, bone age according to Greulich and Pyle (9) assessed by individual physicians, pubertal stage (10, 11), and parents heights. Dates of initiation of treatment were grouped into three periods (19731980, 19811984, and 19851989) after verification of the homogeneity of variables within these periods. SD scores of height and weight for age were calculated (12). The genetic target height SD score was calculated as the average of the parental SD scores, using standards obtained in the late 1960s (13). Patients were coded as pubertal if Tanner stage was P2B2 or more (girls) or P2G2 or more (boys). The etiology of short stature was coded in four categories: craniospinal irradiation-induced GHD, other organic GHD or multihormonal deficiencies, idiopathic GHD, and short stature without GHD.
GH stimulation tests
The results of two GH stimulation tests performed using national guidelines had to be provided to Association France Hypophyse to initiate GH treatment. The various stimuli used were: arginine, clonidine, L-DOPA, glucagon, insulin-induced hypoglycemia, ornithine, GHRH, arginine- and insulin-induced hypoglycemia, clonidine and betaxolol, glucagon and betaxolol, glucagon and propranolol, or other stimuli. GHRH tests were maintained in this analysis, although normal GH peaks during this test are much higher than those after conventional pharmacological tests (14). GH measurements were performed by local laboratories. The peak GH value during the test was recorded even if the peak value was observed at the baseline measurement.
Criteria used for the attribution of GH treatment
As only the data from patients ultimately treated with GH are
analyzed and reported here, the criteria used to decide the attribution
of GH treatment influenced our results. At the beginning of the
collection of this dataset (1973), two peak GH values less than 5 ng/mL
(complete GHD) were required; from 1980 onward, the threshold was
progressively moved to 10 ng/mL (partial GHD). After the description of
GH neurosecretory dysfunction (15), patients with peak GH of 10 ng/mL
or more and subnormal spontaneous GH secretion were considered GH
deficient (Table 1
). A small number (n = 68; 2%) of non-GHD
children were treated on a compassionate basis.
Statistical methods
Intraclass correlation coefficients (r) and their 95%
confidence intervals were computed to analyze the reliability of GHST
(16). The intraclass correlation coefficient expresses the relative
magnitude of the two components of total variability, i.e.
biological variability (between-subject variability,
2BS) and random error (method error,
2ER), in a series of measurements in
different subjects. The relationship between the measurement error and
its true value, estimated by the mean, was illustrated as described
previously (17).
To identify factors associated with peak GH during GHST, we randomly selected one test for each patient to avoid analyzing duplicates of patients data (two GHST for the same patient). A univariate analysis was initially performed after adjustment of the calendar year of the test and the diagnosis. Factors significantly associated with peak GH in the univariate analysis were then used to construct a multivariate regression model in several stages. First, the model had to take into account the calendar year of testing, as criteria for initiation of treatment varied with time. Second, the etiology of GHD was introduced. Third, information about the child (chronological and bone age, height and weight SD scores, and puberty) were introduced in the model as well as genetic target height SD score. Last, the nature of the stimulus used in the GHST was added to significant predictors of the preceding stage. Categorial variables were dichotomized (equal to 1 if the condition was true or to 0). The computations were performed with the SAS statistical package (SAS Institute, Cary, NC) (18).
| Results |
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Five of the 11 different stimuli were used in more than 80% of the 6373 tests: ornithine, arginine- and insulin-induced hypoglycemia, insulin-induced hypoglycemia, arginine, and glucagon and propranolol. Of the 66 theoretical pairs of tests (not counting other tests), 62 were actually used at least once. The most frequent association (ornithine/insulin-induced hypoglycemia) was used in 12.7% of the patients, and the 10 most frequent associations of tests encompassed 70.5% of the patients.
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Reliability was calculated for patients who had undergone the same
test twice, for the most frequent associations of tests, and for all
pairs of tests (reliability of any pair of tests used). Intraclass
correlation coefficients (r) and 95% confidence intervals are
presented in Fig. 1
. In all cases, r was inferior to 0.8, a commonly
admitted threshold for satisfactory reliability. As expected, r was
generally higher in patients who had the same test twice than in
patients who had two different tests. Figure 2
displays this
variability for six of the pairs of tests and allows the evaluation of
reliability at various levels of plasma GH. Reliability appeared good
for very low peak GH values and decreased with increasing GH values.
The 95% confidence interval of the difference between the two tests is
between -5 and 5. This indicates that patients with a mean peak
plasma GH of 5 ng/mL could have values of approximately 0 and 10 ng/mL
as well as two values around 5 ng/mL. The losangic shapes of the plots
suggests an improvement of reliability around peak GH values of 10
ng/mL. This artifact is due to the inclusion in this cohort of 92% of
patients with two GH peaks below 10 ng/mL; there is little room for
variability when a patient has an average and a maximum GH peak close
to 10 ng/mL.
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In the univariate analysis, the following parameters were
associated with peak GH: age, bone age, puberty, height SD
score, weight SD score, genetic target height
SD score, and nature of the pharmacological stimulus. These
factors were then used to construct the multiple regression model
presented in Table 4
. As expected, peak GH increased
with the calendar year of diagnosis of GHD, subdivided into three time
periods. Diagnosis was an independent predictor of peak GH, with
organic GHD or multiple pituitary deficiencies associated with the
lowest GH values and idiopathic GHD associated with the highest, with
the exception of non-GHD patients. Several clinical characteristics of
the patients were independently associated with peak GH in our model.
Height SD score was positively associated with peak plasma
GH. Height SD score had a ß coefficient of 0.47,
indicating that when height decreased by 1 SD score unit,
peak GH decreased by 0.47 ng/mL. Bone age and puberty were
independently associated with peak GH. Age itself, associated with peak
GH in the univariate analysis, was not an independent predictor of peak
GH in the multivariate model due to its interaction with bone age.
Genetic target height SD score had a negative ß,
indicating that patients with shorter parents secreted more GH. Body
weight SD score was also negatively associated with peak
GH, indicating that overweight patients had lower peak GH responses.
Last, the nature of the stimulus was a strong independent predictor of
peak GH response, with ß coefficients ranging from -0.80 to 1.43
(excluding the GHRH test). As these results had been obtained after
randomly selecting one test per patient (see Subjects and
Methods), we performed the same analysis on the other half of the
dataset; essentially similar results were obtained for all variables,
including the nature of the pharmacological stimulus, except for rarely
used tests (tests with n < 162; Table 4
). Therefore, after
correction for all other parameters, the nature of the stimulus could
alter peak GH by approximately 2 ng/mL. It should be underlined that
the predictive parameters identified here only accounted for 19% of
the variance of peak GH.
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| Discussion |
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Over the years, several GH secretion tests have been developed using single or multiple pharmacological stimuli (1, 3, 4). Our results confirm a very heterogeneous use of these stimuli; in the Kabi International Growth Study, 32 different tests were used in 3896 children, with wide variations between countries (19). Although all of these agents influence GH through hypothalamic GHRH secretion (20), some of them are considered stronger than others (21), but their relative potencies have never been systematically examined (22, 23). Reports from the Kabi International Growth Study (19) and by Rochiccioli et al. (24) have shown differences between stimuli, but no correction was made for other variables influencing peak GH. In our multivariate analysis, the nature of the stimulus was independently associated with peak GH, and patients tested with arginine had, on the average, 1.88 ng/mL lower peak GH values than those tested with glucagon-betaxolol. As GH secretion seems to be continuously distributed between normal and idiopathic GHD, the choice of the stimulus can influence the diagnosis if a single cut-off value is used.
Several clinical parameters were also independently associated with peak GH, namely bone age, height SD score, puberty, weight SD score, and genetic target height SD score. Younger, prepubertal patients with severe height deficits and tall parents were more likely to have low peak GH values. The correlations of GH secretion or plasma insulin-like growth factor I (IGF-I) with height SD score have been previously reported (25, 26, 27). The similar finding made in our dataset reinforces the concept that GH secretion is a quantitative trait that plays a role in the multifactorial variation of growth. The positive association of peak GH with bone age and pubertal status probably reflects the problems in making the diagnosis of GHD around puberty. Older, pubertal patients had higher GH values, consistent with transient early pubertal decrease in GH secretion (28, 29, 30, 31, 32). As discussed previously (33), our findings document the negative impact of adipose tissue on GH secretion and the risk of overdiagnosing GHD in overweight patients. Several hypotheses can be made for the negative association of genetic target height SD score and peak GH. Most likely, patients with borderline criteria for GHD (those with the highest peak GH values) were not GH deficient and had genetically determined short stature. Alternatively, genetically determined quantitative variations in peripheral GH sensitivity could play a role, as demonstrated recently in patients with idiopathic short stature (34). Our analysis only explained 19% of the variance of peak plasma GH, indicating the existence of other unrecognized factors influencing GH secretion.
Finally, our study also allowed evaluation of the reliability of GHST.
All intraclass correlation coefficients (r) were below 0.8, indicating
poor reliability. It should be stressed that our results overestimate
the reliability of GHST performed in children evaluated for GHD,
because patients treated for GHD were selected for concordant peak GH
values. The data presented in Fig. 2
clearly depict the difficulty in
defining any cut-off value for peak plasma GH due to the very wide
intrasubject variability.
Last, our study has several methodological limitations. First, GH tests and measurements were performed at a large number of centers with different laboratory methods over a period of almost 2 decades. Due to the various reagents and standards used over this period, GH levels, expressed in nanograms per mL, actually represent variable amounts of active GH (35, 36). Second, we could only analyze data from patients treated with GH and had no access to patients evaluated for possible GHD. Third, the criteria used to include patients in this study (i.e. the decision to initiate GH treatment) were not uniform over time. However, the introduction of the calendar year in the multivariate analysis corrects the impact of the evolution of medical practices. Fourth, our results describe French practices that might be different from those used in other countries. However, the centralized database of the Association France-Hypophyse presented the unique opportunity of performing an exhaustive analysis of GH-testing practices in a given country over almost 2 decades.
The conclusions that can be derived from our study are in line with recent recommendations for diagnosis and treatment of GHD (4, 6). We should select a very limited number of GHST or perhaps just a single powerful one (glucagon and propranolol or glucagon and betaxolol from our study). The lack of reliability of a GHST should be taken into account and might be improved by performing the same test twice rather than two different ones in a given patient. By decreasing the number of tests, we will obtain more data on reliability and distribution in short children evaluated for possible GHD, and we should try to obtain normal age-matched values (37, 38). Last, alternative or complementary approaches to the diagnosis and definition of GHD should be developed, including the reliance on GH-dependent proteins such as IGF-I and IGF-binding protein-3 or increased reliance on auxological parameters (8).
| Acknowledgments |
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Received October 22, 1996.
Revised February 26, 1997.
Accepted April 18, 1997.
| References |
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