From the Guidelines
A blood test z-score is a statistical measurement that shows how far a specific blood test result deviates from the population average, calculated as z score = (observed value - predicted mean value) / RSD, where RSD is the residual SD of the reference population 1. When your blood test result is converted to a z-score, it indicates how many standard deviations your value is above or below the mean for people of similar characteristics. A z-score of 0 means your result is exactly at the average, while positive z-scores indicate above-average results and negative z-scores indicate below-average results. For example, a z-score of +2.0 means your result is two standard deviations above the mean, placing it in approximately the 97.5th percentile. Z-scores are particularly useful in laboratory medicine because they standardize results across different tests and laboratories, allowing for more meaningful comparisons. They help healthcare providers determine if a result is truly abnormal or just slightly outside typical ranges, which aids in clinical decision-making about whether further investigation or treatment is necessary. This standardized approach is especially valuable when monitoring trends in blood values over time or when comparing results from different testing facilities, as seen in studies such as 1 where z-scores were used to compare blood pressure values in children. Key points to consider when interpreting z-scores include:
- The reference population used to calculate the z-score
- The standard deviation of the reference population
- The clinical significance of the z-score value
- The potential for z-scores to be influenced by factors such as age, sex, and weight. In clinical practice, z-scores can be used to identify individuals with abnormal blood test results, monitor changes in blood values over time, and compare results across different laboratories and testing facilities. For instance, a study published in 1 used z-scores to compare blood pressure values in children and found that the prevalence of hypertension was higher according to the American Academy of Pediatrics guideline compared with the Fourth Report. Overall, z-scores provide a valuable tool for healthcare providers to interpret blood test results and make informed clinical decisions.
From the Research
Definition of Z Score
- A Z score is a statistical measure that describes a value's relationship to the mean of a group of values [ 2 ]
- It is a way to compare a single data point to the average of a dataset, taking into account the amount of variation in the dataset [ 3 ]
Calculation and Interpretation of Z Score
- Z scores can be calculated by subtracting the mean of a dataset from a single data point, and then dividing by the standard deviation of the dataset [ 2 ]
- Z scores can be used to compare different datasets or to identify outliers in a dataset [ 3 ]
- A Z score of 0 means that the data point is equal to the mean of the dataset, while a positive Z score means that the data point is above the mean, and a negative Z score means that the data point is below the mean [ 2 ]
Application of Z Score in Medical Research
- Z scores can be used in medical research to compare the results of different tests or to identify patients who are at risk for certain conditions [ 3 ]
- For example, a Z score can be used to compare a patient's blood test results to the average results of a healthy population [ 2 ]
- Z scores can also be used to monitor changes in a patient's condition over time, such as changes in blood glucose levels [ 4, 5, 6 ]