The present invention relates to complexes comprising one or more markers and one or more biomolecules for use as a contrast agent in positron emission tomography based imaging. The complexes according to the present invention preferably accumulate in a target compartment, such as the kidney. The present invention further relates to methods for generating the complexes and methods for using the complexes such as for evaluation of different aspects of kidney functionality e.g. for calculation of total and/or regional glomerular filtration rate in the kidney without the need for sampling blood or urine.
