Validity in Screening Tools

Validity refers to the extent to which a tool actually measures a trait. Does the tool actually assess what it purports to measure?

Screening tools should be validated for the targeted population, setting, and disorder or diagnosis.

Importance of validity when selecting a screening tool

Validity is important because validated tools have been proven to measure what they claim to measure in a specific population. A validated tool detects those who truly have the condition.

ADVANTAGES OF VALIDATED TOOL

More likely to correctly identify clients who truly need help
Less likely to miss those who need intervention
Reduced waste of limited resources on those who do not truly need intervention
Helps demonstrate effectiveness related to positive health outcomes.

More likely to get the desired outcomes when using validated tools

as the appropriate persons are targeted

Components of validity

Validity is measured by sensitivity and specificity and by predictive value, which range from 0 – 100%.
Positive and Negative Predictive Values incorporate sensitivity and specificity and are more useful to clinicians because they also consider the population being tested. Ideally, a screening test should be highly sensitive and highly specific and have both positive predictive value and negative predictive value.

The tool should also be reliable or give the same result every time it is used, and it should have high inter-rater reliability when more than one person gives the test.

Sensitivity	How effective the tool is in detecting a disease or condition in those who have the disease or condition. The higher the sensitivity of a nutrition screening tool, the fewer cases of nutrition risk that go undetected.
Specificity	The extent to which a test gives negative results in those who are free of the disease or condition. The higher the specificity of a nutrition screening tool, the fewer well-nourished persons are incorrectly labeled as at nutrition risk, and the fewer resources are wasted on those who need no intervention.
Positive predictive value	How many of the subjects who test positive truly have the disease.
Negative Predictive Value	How many of the subjects who test negative truly do not have the disease.
Reliability	Assures that results obtained by different investigators are consistent when repeated in the same subjects. ¹¹

Validity of the MNA®

The MNA^® is the most well validated screening tool for the elderly. The original validation study on the full MNA^® demonstrated the MNA^® had a sensitivity of 96%, specificity of 98% and positive predictive value of 97% compared to clinical status. ²¹ The original MNA^®-SF has a sensitivity of 98%, specificity of 100%, and diagnostic accuracy of 99% for predicting undernutrition. ¹⁸ The sensitivity and specificity of the recently revised MNA^®-SF is almost identical to the original MNA^®-SF, confirming the original MNA^®-SF is valid and compares well against the full MNA^®.⁴²⁰