Creatinine excretion in children and the usefulness of creatinine equivalents in amino acid chromatography

131

CLINICA CHIMICA ACTA

CREATININE

EXCRETION

CREATININE

EQUIVALENTS

D. A. APPLEGARTH, Jlepartments

IN AMINO

D. F. HARDWICK

ofPediatrics and

AND ACID

THE

USEFULNESS

OF

CHROMATOGRAPHY

.&ND P. 31. ROSS

Univevsity o,f British Columbia a*Ld Vancouver 9. B.C. (Canada)

Pathology,

The Vancouver General Hospital, (Received

IN CHILDREN

June 3, 1965)

SUMMARY

The daily output of urinary creatinine and a-amino nitrogen was estimated using samples from 228 children aged between one month and five years. The data demonstrate the marked variability from child to child in the output of both of these substances. The data also indicate deficiencies in the practice of using creatinine equivalents as a basis for quantitative examination of urinary amino acids in children.

INTRODUCTION

It is common practice in some laboratories to perform chromatography of urinary amino acids by applying to each chromatogram a volume of urine equivalent to a fixed amount of creatinine (“creatinine equivalent”)l+‘. This is done in an attempt to apply to chromatogranls a standard amount of the daily renal output of amino acids thus providing a standard for comparison of chromatograms from different individuals. In the course of performing urinary amino acid chromatography using the technique of “creatinine equivalents” faint patterns of amino acids were often obtained. To determine if this were the result of variation in the daily excretion of creatinine among children of the same age, the creatinine content and the alpha(E-) amino nitrogen in 24-h urines of normal children were checked. From this study, evidence of deficiencies in the practice of using the “creatinine equivalents” of urine as a basis for amino acid chromatography are presented. METHODS

All the urine excreted over a 24-h period was collected from children who had been hospitalized for a variety of diseases. For children less than two years old collections were made on a metabolic bed with the urine collected directly into a container encased in ice. Contamination with fecal material was avoided by imrnobilizing children on their stomachs and using a half diaper. No children who had any clinical or biochenlical evidence of renal or liver CLin. C&m. ilcta, 22 (1968) 131-134

132

APPLEGARTH

&al.

disease were included in the study. Urine amino acid chromatograms from all the children in the study showed no evidence of disturbed amino acid metabolism. Children were on a normal diet during the collections. Creatinine was measured by the Jafftl reactions. The a-amino specimens was measured by the method of Khachadurian et aL4.

nitrogen

in urine

RESULTS

The data were grouped according to the age of the children and the relationship of amino acid excretion to creatinine excretion was explored. The results are shown in Tables

I, II, and III.

DISCUSSION

The average amount of a-amino

nitrogen

excreted

per mg creatinine

at

yr

s-10

is almost half the average amount for the I month to I year age group (Table I) and there is wide variation in this ratio from person to person. A decrease in the ratio of a-amino TABLE

nitrogen

output with age means that less a-amino

nitrogen

RELATED

FOR

OF AGE

C(-AMINO

NITROGEN

TO

A FIXED

AMOUNT

OF

CREATININE

CHILDREN

GROUPS

0.47* 0.35* 0.24*

50 42 34

rnO.-I yr. I ?‘r.-5 5’1. 5 yr.-10 yr. I

* These means differ significantly ** * I Standard Deviation. TABLE

will

I

EXCRETION OF THREE

to creatinine

& 0.21** & o.rO** + 0.10**

from each other (P < 0.01)

II

DAILY CREATININE EXCRETION

of

Xumber children studied

I-2 2-3

3-4 4-5 TABLE EXCRETION

Children studied

* f

(mgl24

96 59 32 26 14

O--I

96

Range ofcreatixine excretion

Percentage of dai1.v cveatinine output associated with 1 mg creatinine

h.I

25-150 70-2220

0.7-4.0 0.4-1.4

IOO-250

0.4-I

100-300 100-300

0.3-r 0.3-1

III OF

CREATININE

AND

WAMINO

Creatinine excretion

wAmino nitrogen

(mgl24

(mgl24

h)

90 :k 4r* I Standard

42 *

Deviation.

C&z. C/him. Acta, 22 (1968) 131-134

NITROGEN

FOR

CHILDREN

Creatinine excretion hl

19.6*

(mglkgl24 12.0

*

h)

2.9*

AGED

LESS

THAN

ONE

YEAR

CREATININE

EXCRETION

I33

IN CHILDREN

be placed on chrolnatograms in older children if “creatinine equivalents” are used. Despite this, a case could be made for the use of “creatinine equivalents” in amino acid chromatography if creatinine excretion were similar from child to child and application of a standard amount of creatinine indicated that a standard percentage of the daily renal output of creatinine by each child would be applied to each chromatogram. Table II shows that creatinine output varies from person to person six-fold for the I month to I year group and three-fold for other age groups. It is apparent that the application of a “creatinine equivalent” of urine will result in different percentages of the daily renal output being applied to a chromatogram. The difficulties encountered in interpreting the chromatograms are obviously accentuated in the children under 12 months of age. Table III shows the variability of creatinine and a-amino nitrogen excretion in more detail for children less than 12 months of age. There is a wide variation in daily creatinine and cc-amino nitrogen outputs. However, when the values are related to body weight the creatinine output per day per unit body weight is much more constant than the daily output of creatinine. This fact indicates that the other examples of variability discussed above are related mostly to individual metabolic variation and less to the recognized difficulty of obtaining complete daily urine collections in young children. The data reported here on variation of creatinine output from individual to individual complement two earlier reports 5~6.Paterson5 reported on the day to day variability of creatinine excretion in eight healthy adults. Ghadimi and Schwa&mans reported on variations in the ratio of ix-amino nitrogen to creatinine output in 34 children. These latter authors did not, however, measure variability of creatinine excretion alone. Both reports indicate that the use of “creatinine equivalents” is of doubtful validity. If this method is adopted, then great caution should be used before a faint amino acid pattern is dismissed as normal or a heavy pattern is interpreted as an aminoaciduria. The most satisfactory way to evaluate amino acid excretion is to use z4-hourly urine collections of which a fixed aliquot containing a standard amount of a-amino nitrogen is applied to each chromatogram. By knowing the a-amino nitrogen it is possible to detect a genuine increase in daily amino acid output. In addition, all chromatograms will then contain the same amount of ninhydrin-positive material, and variations of output throughout the day will not affect the resulting chromatograms. While the use of “creatinine equivalents” will probably continue to be used for the purpose of screening children for specific amino acidopathies the practice of reporting outputs of amino acids as a function of unit amounts of creatinine should be discouraged. Without the assistance of Dr. H. G. Dunn this study would not have been possible. REFERENCES I H. K. BERRY, Pediatrics, 25 (1960) 983. 2 W. RICHARDS, G. N. DONNELL, W. A. Wr~sox, ~~~~ld~e~~, rag (1965) 185,

D. STOWENS

AND T. PERRY, Am. J. lkenses

A~LEG.~RTH

I34 3 E. J. KING AND I. D. P. WOOTTON,

Micro-Analysis

1959, p. 161. 4 A. KHACHADURIAN, TV. E. KNOX 5 N. ~"ATERSON, Clin. Chim. Acta, 6 H. GHADIMI AND H. SCHWACHMAN,

18 (1957) 57. Am. J. Diseases

Clips. Chim.

Acta,

2~ (1968)

I~I-134

AND

A.

&I. CULLEN,

in Medical J.

Riochemistr.~,

Lab.Clin.

Childmz,

Churchill,

Awed., 56(rg60)

gg (1960)

457.

et al. London,

321.