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A reference range is usually defined as the set of values 95 percent of the normal population falls within (that is, 95% prediction interval). [2] It is determined by collecting data from vast numbers of laboratory tests. [citation needed]
The standard definition of a reference range for a particular measurement is defined as the interval between which 95% of values of a reference population fall into, in such a way that 2.5% of the time a value will be less than the lower limit of this interval, and 2.5% of the time it will be larger than the upper limit of this interval, whatever the distribution of these values.
Cortisol follows a circadian rhythm, and to accurately measure cortisol levels is best to test four times per day through saliva. An individual may have normal total cortisol but have a lower than normal level during a certain period of the day and a higher than normal level during a different period.
Average respiratory rates vary between ages, but the normal reference range for people age 18 to 65 is 16–20 breaths per minute. [4] The value of respiratory rate as an indicator of potential respiratory dysfunction has been investigated but findings suggest it is of limited value.
Plasma renin activity (PRA), also known as the renin (active) assay or random plasma renin, is a measure of the activity of the plasma enzyme renin, which plays a major role in the body's regulation of blood pressure, thirst, and urine output. Measure of direct renin concentration (DRC) is technically more demanding, and hence PRA is used instead.
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To confirm inappropriately low cortisol secretion, testing can include baseline morning cortisol level in the blood or morning cortisol level in the saliva. [2] Cortisol levels typically peak in the morning; thus, low values indicate true adrenal insufficiency. [2] Urinary free cortisol can also be measured, but are not necessary for diagnosis. [2]
Traditionally, cortisol and ACTH levels (separate lavender top tube) are drawn at baseline (time = 0). Next, synthetic ACTH or another corticotropic agent is injected IM or IV, depending on the agent. [12] Approximately 20 mL of heparinized venous blood is collected at 30 and 60 minutes after the synthetic ACTH injection to measure cortisol levels.