Urinary 3-methylhistidine/creatinine ratio as a clinical tool: correlation between 3-methylhistidine excretion and metabolic and clinical states in healthy and stressed premature infants.

We have investigated the role of the urinary 3-methylhistidine (3MH) excretion, a measure of protein catabolism, in the evaluation of the metabolic state of premature infants. Two-hundred and twenty-two 24 hr urine collections and 3MH/Cr ratio determinations (expressed as mumoles of 3MH per mg creatinine) were carried out in 36 infants (average gestational age 32.7 +/- 0.7 wk, weight 1640 +/- 120 grams) and the relationship between the 3MH/Cr ratios and the metabolic and clinical state has been investigated. Five or more 3MH/Cr measurements were carried out on each of 19 infants and serial determinations on four of those babies are presented. The urinary 3MH/Cr ratio of healthy infants with adequate caloric intake and normal growth curve was .148 +/- .039 (S.D.) mumol/mg, about 35% higher than the 3MH/Cr ratio in healthy adults. As long as the premature infants were healthy the degree of prematurity had no effect on the 3MH/Cr ratio. The relationship between 3MH/Cr ratio and nitrogen balance was highly significant (p less than .001). 3MH/Cr ratio also correlates very well with the metabolic status of the infants: in the group with normal 3MH/Cr ratios less than or equal to .175 (.148 + 1 S.D., n = 90) there were four clinically stressed infants (4.4% false negative rate) while in the group with elevated 3MH/Cr ratios greater than .225 (.148 + 2 S.D.; n = 79) there were only three clinically well infants (3.8% false positive rate). In comparing the clinical status and 3MH/Cr ratios, we found that in the group of infants who could not be clearly defined as clinically well or stressed (n = 108) the 3MH/Cr ratio was more useful than clinical judgment in the prediction of metabolic status. It can be concluded that 3MH/Cr ratio is a potentially useful clinical tool which describes with high accuracy the clinical and metabolic status of premature infants. This conclusion is further supported by the data of serial 3MH/Cr determinations.