What is the clinical use of z scores in the Neonatal Intensive Care Unit (NICU)?

Clinical Use of Z-Scores in the NICU

Z-scores in the NICU are essential standardized measurements that account for gestational age and growth variations, primarily used for nutritional assessment, cardiac function evaluation, and identifying infants at risk for adverse outcomes.

Nutritional and Growth Assessment

The European Society of Pediatric and Neonatal Intensive Care (ESPNIC) recommends measuring weight, height/length, mid-upper arm circumference, and head circumference (in children <36 months) on admission and regularly throughout NICU stay, expressing all measurements as z-scores. 1

• Z-scores for body mass index-for-age (or weight-for-length in infants <2 years) should be used to screen for patients at extremes of these values, as malnutrition is associated with longer ventilation periods, higher infection risk, prolonged NICU stay, and increased mortality 1
• Nutritional status should be reassessed at least weekly throughout hospitalization, as patients are at risk of nutritional deterioration that adversely affects clinical outcomes 1
• Z-scores are superior to percentages of predicted values because they account for interindividual variability and allow tracking of changes over time, providing more clinically meaningful information than simple reference range comparisons 1

Cardiac Function Evaluation via Point-of-Care Ultrasound (POCUS)

ESPNIC strongly recommends using age-specific z-scores for cardiac measurements in neonates, as absolute values vary significantly with age and body size. 1

Key cardiac parameters requiring z-score interpretation include:

• Left ventricular output (stroke volume): Z-scores must be used for different ages; neonates typically 150-400 ml/kg/min 1
• Mitral annular plane systolic excursion (MAPSE): Z-scores should be applied; term neonates typically >8 mm (8-11 mm range) 1
• Right ventricular output: Z-scores available for different ages should be used; neonates typically 150-400 ml/kg/min 1
• Tricuspid annular plane systolic excursion (TAPSE): Term neonates typically >8 mm (8-11 mm); z-scores available for children 1

Risk Stratification and Prognostication

Birth weight z-scores are independent predictors of mortality and morbidity in infants undergoing cardiac surgery, with even mildly reduced z-scores (-1.0 to -0.5) conferring increased risk. 2

• Severe growth restriction (birth weight z-score -4 to -2) increases operative mortality 2.4-fold compared to reference populations 2
• Moderate growth restriction (z-score -2 to -1) increases mortality 1.7-fold 2
• Even mild growth restriction (z-score -1.0 to -0.5) increases mortality 1.4-fold 2
• The strongest association between poor fetal growth and operative mortality exists in early-term infants, with significant interaction between birth weight z-score and gestational age 2

Interpretation and Clinical Application

Z-scores indicate how many standard deviations a measurement deviates from the population mean, with values below -2 typically warranting intervention consideration. 1

• A z-score of 0 represents the population mean, with each unit representing one standard deviation 1
• Z-scores below -2 (approximately 5th percentile) or above +2 (approximately 95th percentile) are considered clinically significant 1
• Serial z-score measurements are more valuable than single measurements, as they track growth trajectory and response to interventions 3
• Z-scores allow comparison across different parameters and time points, facilitating comprehensive assessment of infant status 3

Common Pitfalls to Avoid

• Never use absolute measurements alone without converting to z-scores in neonates, as normal values vary dramatically with gestational age and size 1
• Ensure appropriate reference populations are used that match the infant's gestational age and ethnicity to avoid misinterpretation 1
• Do not rely solely on percentages of predicted values, as these do not account for the normal distribution's variability 1
• Recognize that z-scores for height-for-age may be more clinically important than mean values, as being below the 5th or 10th percentile is associated with increased childhood morbidity and mortality 1