Editor,—The paper by Lamont and colleagues1 is a valuable survey of the findings associated with mitochondrial DNA mutations. However, DNA analysis is time consuming and expensive, and there is a need for a screening test to identify those who merit further investigation. Measurement of lactate has been proposed, and it was disappointing that this study found it lacked specificity. However, two issues need discussion.

Lamont et al state (table 2) that 11 of 13 patients had raised lactate concentrations in blood or cerebrospinal fluid (CSF); however, the data they present refers only to plasma concentrations.1 Lactate is more stable in CSF than in plasma,2 ,3 and …