A first tracer agent for use in an in vivo diagnostic method for diagnosing a patient's renal function, wherein the method comprises: delivering a bolus of the first tracer agent into the patient's vasculature, wherein the first tracer agent is at least substantially cleared by healthy renal function and emits a first fluorescent light at a first emission wavelength in response to excitation by a first excitation light at a first excitation wavelength, transmucosally applying the first excitation light to superficial vasculature of the patient at a first mucosal monitoring site to excite the first tracer agent in the superficial vasculature at the first mucosal monitoring site such that the first tracer agent emits the first fluorescent light; transmucosally receiving the first fluorescent light emitted from the first tracer agent at the first mucosal monitoring site; transforming the received first fluorescent light to produce a first fluorescence signal representative of a first fluorescence of the first fluorescent light; processing the first fluorescence signal to determine a first decay rate of the first fluorescent light; and processing the first decay rate of the first fluorescent light to determine a glomerular filtration rate of the patient, wherein the first mucosal monitoring site includes a first mucosal excitation site and a first mucosal detection site, characterised in that, the first mucosal monitoring site includes a tympanic membrane, and in that the first mucosal detection site is positioned downstream from the first mucosal excitation site according to the flow direction of the blood in a first mucosal vessel encompassed by the first mucosal monitoring site. delivering a bolus of the first tracer agent into the patient's vasculature, wherein the first tracer agent is at least substantially cleared by healthy renal function and emits a first fluorescent light at a first emission wavelength in response to excitation by a first excitation light at a first excitation wavelength, transmucosally applying the first excitation light to superficial vasculature of the patient at a first mucosal monitoring site to excite the first tracer agent in the superficial vasculature at the first mucosal monitoring site such that the first tracer agent emits the first fluorescent light; transmucosally receiving the first fluorescent light emitted from the first tracer agent at the first mucosal monitoring site; transforming the received first fluorescent light to produce a first fluorescence signal representative of a first fluorescence of the first fluorescent light; processing the first fluorescence signal to determine a first decay rate of the first fluorescent light; and processing the first decay rate of the first fluorescent light to determine a glomerular filtration rate of the patient, wherein the first mucosal monitoring site includes a first mucosal excitation site and a first mucosal detection site, characterised in that, the first mucosal monitoring site includes a tympanic membrane, and in that the first mucosal detection site is positioned downstream from the first mucosal excitation site according to the flow direction of the blood in a first mucosal vessel encompassed by the first mucosal monitoring site.