Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1303 page)

   Interfering substances: Lipemia, clotted or hemolyzed blood, blood collected in wrong anticoagulant, or improperly filled test tube.

   
VWF antigen:
normal values: 60–150. The antigen values may be higher than the vWF activity in certain subtypes with qualitative defects, resulting in a ratio <0.7 of activity/antigen.

   
Factor VIII coagulant:
normal range: 70–150%

   
RIPA
is a semiquantitative assay for VWF, used when there is a strong suspicion for VWD. It uses ristocetin as the platelet agglutinating agent in the presence of VWF. The changes in optical density (OD) are recorded in an aggregometer. Abnormal response to ristocetin is the result of von Willebrand disease or of the platelet receptors responsible for binding von Willebrand factor

   Use of RIPA

   To estimate VWF activity and rule in or out type 2B VWD (see below.)

RESULTS

   Two concentrations of ristocetin are used in the RIPA assay: a high concentration results in
65% or more change in OD
if normal VWF is present. A lower concentration of ristocetin is also used: it will not agglutinate platelets in cases with normal (or low) VWF but will result in agglutination in cases of type 2B VWD, which represent a gain of function in the factor. A similar pattern is found when platelets form patients with platelet-type VWD (pseudo-VWD) are used.

   Limitations

   The assay is labor intensive and requires highly trained technologists.

   The quantitation of VWF by this assay is imprecise.

   Clotted blood or blood obtained in an appropriate anticoagulant invalidates results.

WATER DEPRIVATION TEST

   Definition

   Normal physiologic response to the water deprivation will increase plasma osmolality, which will then lead to a progressive elevation in ADH release and an increase in urine osmolality. Once the plasma osmolality reaches 295–300 mOsm/kg (normal: 275–290 mOsm/kg), the effect of endogenous ADH on the kidney is maximal. At this point, administering ADH does not further elevate the urine osmolality unless endogenous ADH release is impaired (e.g., the patient has central diabetes insipidus [DI]). This test is also known as the water restriction test.