Glutaric acidemia type I (GA1) is associated with elevated glutarylcarnitine (C5DC), typically measured as its butylester by acylcarnitine profile analysis using tandem mass spectrometry (MS/MS) and the precursor-product ion pair of m/z 388-85. This method neither distinguishes between C5DC and its isomer 3-hydroxydecanoylcarnitine (C10-OH) nor reliably detects the low-excretor variant of GA1, leading to both false-positive and false-negative results when testing for GA1. To overcome these limitations, we developed an LC-MS/MS method that discriminates C5DC from C10-OH by the use of precursor-product ion pairs specific for butylated C5DC (m/z 388-115) and underivatized C10-OH (m/z 332-85). The C5DC method was validated over the linearity range of 0.025-20 μM with a lower limit of quantification (LOQ) of 0.025 μM. Excellent precision and accuracy were also observed. We tested plasma samples from 10 patients with confirmed GA1 (including 3 with the low-excretor variant), 21 patients with mild elevations of C5DC or C10-OH by routine acylcarnitine analysis for which GA1 ultimately was excluded, and 29 normal controls. By using the m/z 388-115 ion pair, all cases of GA1, including the low-excretor variant, were reliably distinguished from normal controls. By using the m/z 388-85 pair, patients with ambiguous elevations of C5DC or C10-OH demonstrated clearly elevated levels of C10-OH (m/z 332-85) but normal C5DC (m/z 388-115), confirming that the apparent elevation of C5DC is due to interference by C10-OH. Our method results in excellent detection of GA1, including the low-excretor variant, and also provides a means to discriminate C5DC and C10-OH in follow-up testing and routine acylcarnitine studies.