Thank you for raising this important issue.
While there is evidence to suggest that nitrofurantoin retains clinical efficacy for cystitis with renal function of CrCl < 60 mg/dL, the main issue in patients with renal insufficiency is pulmonary toxicity. A recent trial published in the European Journal of Clinical Pharmacology suggests that despite maintaining clinical cure with reduced clearance, adverse events increased when nitrofurantoin was utilized in patients with a eGFR below 50 mL/min. The incidence of pulmonary adverse events leading to hospitalization increased by a hazard ratio of 4.1 (95% CI 1.31-13.09) to 0.02 per 1,000 person days. Based on these results, nitrofurantoin cannot be recommended for use when eGFR is below 50 mL/min. In addition, it’s important to recall that the absence of evidence is not evidence of safety. The manufacturer's recommendation change for minimum creatinine clearance may reflect unpublished data held by the drug company. Without evidence of safety below recommended dosing guidelines and published evidence of increased harm, the safest approach is to NOT use nitrofurantoin below the manufacturer recommended threshold of CrCl 60 mg/dL. References: Geerts et al. "Ineffectiveness and adverse events of nitrofurantoin in women with urinary tract infection and renal impairment in primary care.", European Journal of Clinical Pharmacology (2013) 69:1701-1707 http://www.ncbi.nlm.nih.gov/pubmed/?term=Ineffectiveness+and+adverse+events+of+nitrofurantoin+in+women+with+urinary+tract+infection+and+renal+impairment+in+primary+care