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Metal-on-metal (MoM) prostheses, in which the bearing surfaces are made of a metal alloy, may release metal ions upon wear and corrosion, potentially inducing both local and systemic toxicity. As the systemic cobalt concentration increases with the degree of implant wear, this concentration needs to be monitored as a means of assessing implant function and the risk of adverse effects. Here, we report on the development, validation and application of a method to quantitatively assess these Co concentrations in whole blood, based on the combination of volumetric absorptive microsampling (VAMS) and inductively coupled plasma - mass spectrometry (ICP-MS). This method could allow patients to collect the required samples at home, as VAMS samples are easy to collect and can be transported to the laboratory via regular mail. The extraction procedure utilized an alkaline extraction mixture with yttrium as internal standard and proved to be independent of the hematocrit and age of the VAMS samples. The Co concentrations in the VAMS extracts were measured using quadrupole-based ICP-MS. The analytical method covers a range of 2–300 μg/L and displays excellent accuracy (bias ≤4%) and imprecision (relative standard deviation ≤ 5% and ≤15% at the lower limit of quantitation (LLOQ)). The method was applied to venous VAMS samples of MoM prosthesis patients (n = 78), yielding promising results. The comparison of these results with those obtained on the corresponding liquid whole blood samples, showed a correlation coefficient of 0.99 and 87% of the data fulfilled the criteria proposed by the Royal College of Pathologists of Australasia (RCPA). Image 1 • Cobalt can be reliably quantified in volumetric absorptive microsamples (VAMS). • The extraction is simple, semi-automated and hematocrit and VAMS age-independent. • Cobalt was measured using a single quadrupole ICP-MS, suitable for routine use. • This method was thoroughly and successfully validated. • VAMS and liquid whole blood-based cobalt concentrations proved to be comparable. [ABSTRACT FROM AUTHOR]