If three hours have passed according to the wall clock, but the machine shows 2.5 hours, what happened?

• A. the machine was in bypass for 30 minutes
• B. the clock was wrong
• C. the patient slept
• D. the machine paused due to alarm

Answer: (A)

The key idea is that the machine’s treatment time tracks only the periods when the dialyzer is actively being used. If the circuit is diverted away from the dialyzer—bypassed—the machine stops counting treatment time, even though real time on the wall clock keeps ticking. Here, three hours have passed according to the wall clock, but the machine shows 2.5 hours, a 30-minute shortfall. That 30-minute gap lines up with a period when the dialyzer was bypassed, so the machine’s timer paused while the wall clock kept running.

Other possibilities are less consistent with the observed mismatch. If the clock were wrong, you’d expect both readings to be off in a systematic way or require clock verification; patient sleep doesn’t affect the machine’s timer, and an alarm pausing the machine would typically be accompanied by an alarm indicator. The straightforward explanation for a 30-minute discrepancy is that the machine was in bypass for 30 minutes.