When is the Z (Z score) used in medical practice?

When Z-Scores Are Used in Medical Practice

Z-scores are used in pediatric and cardiovascular medicine to standardize measurements of cardiac structures, blood vessels, and growth parameters against age- and body size-specific reference populations, allowing clinicians to determine whether a measurement represents normal variation or pathological deviation. 1

Primary Clinical Applications

Pediatric Cardiac Imaging (Most Common Use)

The use of Z-scores is advised for quantification in the pediatric age for all echocardiographic measurements. 1

• Coronary artery assessment in Kawasaki disease: Z-scores ≥2.5 differentiate coronary involvement from fever-related dilation with 98% specificity, with classification ranging from dilation only (Z-score 2 to <2.5) to giant aneurysms (Z-score ≥10) 1, 2
• Aortic root measurements: Z-scores >2.0 indicate aortic dilatation (approximately 98th percentile), with values of 2-3.01-4.0, and >4.0 defining mild, moderate, and severe dilatation respectively 1
• Ventricular dimensions and function: Z-scores are available for left and right ventricular volumes, mass, and functional parameters including newer strain and 3D techniques 1
• Valvular structures and great vessels: Z-scores standardize measurements of valve annuli, pulmonary arteries, and other vascular structures across the pediatric age spectrum 1, 3

The critical advantage is that Z-scores account for the dramatic changes in cardiac dimensions during growth, making absolute measurements alone inadequate and potentially misleading. 4

Neonatal Intensive Care Unit (NICU) Applications

The European Society of Pediatric and Neonatal Intensive Care (ESPNIC) recommends expressing all anthropometric and cardiac measurements as Z-scores in the NICU. 4

• Nutritional assessment: Weight, height/length, mid-upper arm circumference, and head circumference (in children <36 months) should be measured on admission and regularly, with Z-scores <-2 warranting intervention consideration 4
• Point-of-care cardiac ultrasound: Age-specific Z-scores must be used for left ventricular output (neonates typically 150-400 ml/kg/min), MAPSE (term neonates >8 mm), right ventricular output, and TAPSE (term neonates >8 mm) 4
• Growth monitoring: Z-scores for BMI-for-age (or weight-for-length in infants <2 years) screen for malnutrition, which is associated with longer ventilation, higher infection risk, prolonged NICU stay, and increased mortality 4

Pediatric Growth Assessment

Height Z-scores are used to identify growth failure or stunting, typically defined as being below the 5th or 10th percentiles for height-for-age. 5

• Infancy and early childhood (0-5 years): Height Z-scores are highly relevant for detecting failure to thrive and are critical for nutritional interventions to prevent long-term growth deficits 5
• Middle childhood (6-10 years): Height Z-scores remain important for monitoring growth trajectory and identifying children who may benefit from interventions 5
• Persistent Z-scores below -2: For children under 8 years (girls) or 11 years (boys), interventions should be considered for persistent Z-scores below -2 5

Gestational Weight Gain Monitoring

Z-scores provide a method of describing weight gain independently of gestation and make it easier to describe the severity of abnormal weight gain patterns. 1

• Z-scores are less prone to bias and difficulties in interpretation due to nonlinearity compared to raw centiles 1
• They allow adherence to reference distributions, provide a linear scale permitting summary statistics, and have uniform criteria across indexes 1
• Z-scores are useful for detecting changes at extremes of distributions, which is critical for identifying high-risk pregnancies 1

Key Advantages of Z-Scores Over Absolute Measurements

Z-scores describe how many standard deviations a measurement deviates from the population mean, with each unit representing one standard deviation. 4, 6

• Body size correction: Z-scores account for age, sex, height, weight, and body surface area, making them superior to absolute measurements that vary dramatically with growth 1, 4, 6
• Serial tracking: Z-scores provide an excellent means of charting serial measurements over time, allowing detection of growth acceleration or deceleration 6
• Statistical uniformity: Z-scores create a linear scale with consistent statistical properties across different ages and body sizes 1, 4
• Clinical decision thresholds: Z-scores <-2 (approximately 5th percentile) or >+2 (approximately 95th percentile) are considered clinically significant across most applications 4

Critical Pitfalls to Avoid

Never use absolute measurements alone without converting to Z-scores in pediatric populations, as normal values vary dramatically with gestational age and size. 4

• Reference population selection: Ensure appropriate reference populations match the patient's gestational age, ethnicity, and background to avoid misinterpretation 4, 5
• Measurement accuracy: Small errors in coronary artery diameter measurement can translate into larger differences in Z-scores, potentially changing risk categories 1
• Body surface area calculation: Accurate measurement of weight and particularly height is essential for calculating accurate BSA and subsequent Z-scores 1
• Z-score system variability: Different Z-score calculators (Boston, Montreal, DC, Pediatric Heart Network) can yield significantly different results, with coronary lesion rates varying from 24-55% depending on the system used 7
• Nomogram source documentation: Always report the vendor, software, and source of nomograms used, as great variability exists between different Z-score sources 1

Limitations and Gaps in Knowledge

For diastolic parameters, nomograms present limitations because diastolic parameters are less dependent on age and body size. 1

• Dimensional and functional nomograms are lacking for previously pre-term children, low birth weight children, and young athletes 1
• Natural changes in Z-scores occur as children age and "fill out" their body contour, with 70% of normal children showing spontaneous decrease in aortic root Z-score over time 8
• In untreated Marfan patients, 40% showed natural decrease in root Z-score without intervention, raising questions about interpreting Z-score changes as drug effects in clinical trials 8