Background Deficiency of the enzyme lysosomal acid lipase (LAL) presents in infancy with hepatosplenomegaly, adrenal calcification and failure to thrive – Wolman Disease (WD). Cholesterol Ester Storage Disease (CESD) is the more common and milder form and presents later with hepatomegaly and hyperlipidaemia. Until recently there was no recognised method for the measurement of LAL in dried blood spots (DBS).

Aim To develop a simple method for the measurement of lysosomal acid lipase in DBS and validate the technique using samples from patients with CESD.

Methods LAL was extracted from DBS with water and incubated with 4mU-palmitate in acetate buffer and cardiolipin as an activator of LAL. Lalistat 2 is used as an inhibitor of LAL and activity is estimated by measuring total lipase activity and activity in the presence of Lalistat 2. LAL was determined by subtracting activity measured in the inhibited reaction from that in the uninhibited reaction. The assay was performed using 96-well plates and read at 355nm/460nm.

Results LAL activity in normal controls was 0.37 – 2.30 nmol/punch/hr. Samples from CESD patients and WD have significantly reduced LAL activity : <0.04 nmol/punch/hr (n = 32). Activity in samples from obligate carriers is in the range 0.15 – 0.40 nmol/punch/hr (n = 15).

Conclusions Until now, methods for the measurement of LAL required the use of leucocytes and fibroblasts.

These are difficult samples to prepare and methods for the measurement of LAL using these sample types are time consuming and expensive. Measurement of LAL in DBS is made difficult by the presence of other lipases in whole blood. Lalistat 2 is a specific inhibitor of LAL which allows the reliable determination of LAL in DBS. Results show the method differentiates clearly between normal controls, carriers and affected cases.

The DBS sample is simple to prepare and transport. By performing the assay using 96-well plates, samples can be processed efficiently in batches, previously not possible. Therefore it is now practical to screen for WD and CESD in patients with a lower index of suspicion of the disorder.