Hello from JustAnswer.
First, the low eGFR is based primarily on the increase in creatinine. Your age and gender also enters into the calculation, but when the eGFR is significantly lower than a previous reading, it is solely due to an increase in creatinine. So, we only need to discuss the creatinine and BUN.
Any increase in BUN and creatinine could be due to a decrease in kidney function, but there are some systemic conditions that can affect BUN and creatinine. And when the ratio of BUN to creatinine is greater than 20, there is a greater likelihood that this is due to a systemic condition, rather than kidney dysfunction. In your case, the BUN/creatinine ratio is 25, so it is appropriate to consider other systemic causes of an increase in BUN and creatinine.
The most common systemic cause of an increase in BUN and creatinine is dehydration or congestive heart failure. Both can cause an increase in BUN and creatinine by decreasing the blood flow to the kidneys. If there are no other symptoms of congestive heart failure, such as worsening shortness of breath or swelling of the legs, and no findings on exam, the usual approach is to consider whether it is due to hydration status. So, it is appropriate to be drinking good amounts of fluids and recheck the levels. If the levels return to normal, then that is evidence that it was likely due to dehydration.
If I can provide any additional information, please let me know.