Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1060 page)

   A normalization of the clotting time in either test confirms the presence of LA but requires that the tests be repeated in 12 weeks, because frequently the LA is a temporary phenomenon (Fig. 16.1).

   Limitations

   There is considerable interlaboratory variation with the performance of the LA assays, especially dRVVT. In recent surveys, there was a false-positive detection of LAs in 24% of samples and a false-negative result of 18.5% in participating centers.

   One of the factors that may contribute to a false-positive result is contamination with heparin.

   Preanalytical variables, such as improper plasma preparation, may lead to false-negative results because of contamination with platelets.

   It is recommended that assessment for LA not be undertaken while the patient is on oral anticoagulants, if at all possible (see Fig. 16.1).

Figure 16–1
Algorithm for testing lupus anticoagulant antibodies.

Suggested Readings

Giannakopoulos B, Passam F, Ioannou Y, et al. How we diagnose the antiphospholipid antibody syndrome.
Blood.
2009;113:985–994.

Moffat KA, Ledford-Kraemer MR, Plumhoff EA, et al. Are laboratories following published recommendations for lupus anticoagulant testing?
Thromb Haemost.
2009;101:178–184.

LUTEINIZING HORMONE (LH)

See Follicular-Stimulating Hormone (FSH) and Luteinizing Hormone (LH), Serum.

MAGNESIUM (Mg)

   Definition

   Mg is primarily an intracellular ion associated with GI absorption and renal excretion. At least 65–70% of Mg is in ionized state, and approximately 35% serum Mg is protein bound.

   
Normal range:
1.6–2.4 mg/dL.

   
Critical values:
<1.0 and >4.9 mg/dL.

   Use