Performance of low-dose cosyntropin stimulation test in the afternoon

Earlier research on 1 μg low-dose test (LDT) performed using 20.3 cm plastic IV tubing on healthy volunteers, has shown that afternoon testing was associated with a sevenfold increased likelihood of failing the test. Nevertheless, it has been claimed that subnormal cortisol response using plastic tubes might have resulted from cosyntropin adherence to the tube and, thus, loss of the delivered dosage. Following from our previous study, which showed that using a short (2.5 cm) plastic tube does not alter in-vitro-cosyntropin dosage delivery or healthy-volunteers' morning cortisol responses, we predicted that, when using the same short plastic tube, LDT would show comparable morning and afternoon cortisol stimulation. The current study was designed to investigate this prediction by comparing morning and afternoon cortisol responses in healthy volunteers during LDT, using a short plastic tube.

Thirteen healthy adult volunteers were recruited for the study. Each subject underwent morning and afternoon LDT via 25 mm plastic intravenous line tube. Baseline serum cortisol (SC) in addition to SC and salivary free cortisol (SFC) 30-minute responses were determined.

Mean baseline morning SC concentration was higher in the morning than in the afternoon (13.63±3.42 and 9.18±2.78 μg/dL, respectively; P<0.001); however, mean absolute SC-concentration increment between baseline and 30-minute time point was higher in the afternoon than in the morning (11.89±3.50 and 7.71±3.12 μg/dL, respectively; P=0.002). Subsequently, LDT resulted in comparable morning and afternoon 30-minute SC (21.33±3.08 and 21.08±3.43 μg/dL, respectively; P=0.782) and SFC concentration (0.939±0.256 and 1.036±0.372 μg/dL, respectively; P=0.463).

In healthy volunteers, using a 2.5 cm plastic tube, LDT provides comparable morning and afternoon 30-minute stimulated SC and SFC concentration.