Continuing this patient's current antibiotic therapy is the most appropriate management. Creatinine is normally filtered by the kidney from the serum, although a smaller amount is also secreted by the proximal tubule. In patients with more advanced chronic kidney disease (CKD), such as this patient, up to 50% of urine creatinine may be secreted instead of being filtered through the glomerulus. Trimethoprim is known to interfere with creatinine secretion without affecting the glomerular filtration rate (GFR) and can cause increases in serum creatinine of up to 0.5 mg/dL (44.2 µmol/L); this rise in serum creatinine therefore does not reflect a drop in actual kidney function. This effect is reversible upon discontinuation of the medication. Trimethoprim also inhibits the epithelial sodium channel in the collecting tubule, effectively acting as a potassium-sparing diuretic and potentially increasing the serum potassium level. The risk of hyperkalemia is therefore increased when using trimethoprim-containing antibiotics, particularly in patients who are on high-dose trimethoprim, who have underlying CKD that may predispose to hyperkalemia, or who are taking a medication with a potassium-sparing effect (such as an ACE inhibitor or angiotensin receptor blocker). Because of this, trimethoprim should be used with caution and with close monitoring in patients with any of these underlying risk factors who require its use for treating infection. This patient's potassium level is within the normal range, and continued close monitoring during her antibiotic therapy is indicated.

Rifampin may cause red or orange discoloration of the urine but is not associated with a decreased GFR, an increase in serum creatinine, or hyperkalemia.

Urine eosinophil testing by means of special stains has been used classically to diagnose acute interstitial nephritis, which is a diagnostic consideration in a patient taking sulfa drugs. However, testing for urine eosinophils is neither sensitive nor specific for this diagnosis, which recent literature suggests may only be definitively made on the basis of kidney biopsy.