Malonylcarnitine and glutarylcarnitine are important diagnostic metabolites in the screening of dried blood spots by tandem mass spectrometry. The stability of these compounds in spiked blood spots stored at room temperature was studied. Both showed biphasic curves. The malonylcarnitine concentration dropped to 61% in 38 days and averaged 51% +/- 5% for the next 81 days. Glutarylcarnitine dropped to 60% in 42 days and averaged 56% +/- 5% for the next 124 days.