Automated vWF Assay vs. Ristocetin Cofactor Method in Diagnosis of von Willebrand Disease

Group 3 PresentationRobert AckermanSean Cavanagh-VossJennifer LeGonzalo OrtizShea TaylorAutumn Walker

Evidence-Based Clinical Reasoning IUSF Morsani College of Medicine February 2014

Case Description

A 21 year old woman presents to your office for evaluation of abnormal menstrual periods. Her periods last from 7 to 9 days requiring up to 10 pads a day (excessive). On further questioning, she admits to easy bruising and nosebleeds. Family history reveals that her mother has had similar problems.

You suspect von Willebrand disease (vWD). vWD is the most common inherited coagulation abnormality.

Your laboratory informs you of a new automated assay for vW factor (vWF) activity. Since you have not heard about it, you decide to check what the published literature says about this new test and how does it compare to the established ristocetin cofactor method that you used to order to confirm the diagnosis of vWD.

von Willebrand Disease (vWD)

Deficiency in von Willebrand factor (vWF)– Two main functions of vWF:

• Binds and stabilizes factor VIII • Mediates platelet adhesion to vascular injury site

– Three classifications of vWD:• Type 1: partial, quantitative deficiency of vWF• Type 2: qualitative abnormalities of vWF (defective

platelet adhesion or factor VIII binding)• Type 3: near absence of vWF

Diagnosis of von Willebrand Disease

Three main criteria:– Past medical history of excessive bleeding– Family history of excessive bleeding– Laboratory evaluation of vWF

• Screening tests– Platelet count– Activated partial thromboplastin time– Bleeding or closure times

• Confirmatory tests– von Willebrand factor antigen (VWF:Ag)– von Willebrand ristocetin co-factor test (VWF:RCo)– HemosIL von Willebrand Factor Activity Assay

Confirmatory Lab Tests

VWF:RCo– Ristocetin promotes binding of vWF and

platelet glycoprotein Ib – Measures amount and rate of platelet

aggregation– Challenges

• Macroscopic slide method is semi-quantitative• Platelet aggregometer is labor intensive• High inter-assay and inter-laboratory variability

Confirmatory Lab Tests

HemosIL von Willebrand Factor Activity assay– Monoclonal antibody binds to platelet-binding

site of vWF (glycoprotein Ib receptor)– Antibody is absorbed onto latex– Test measures turbidity that results from

agglutination of latex

The Question

P Patients suspected of von Willebrand Disease

I Automated assay for vWF activity

C Ristocetin co-factor test

O Detecting and classifying vWD

Are automated assays done on patients suspected of von Willebrand Disease more or as accurate in the diagnosis of the disease compared with the ristocetin co-factor test?

Search Strategy

PubMed Clinical Queries, accessed through the USF Shimberg Library– Reliable, open database– Provides clinically relevant information that can

help answer case questions Search “von Willebrand factor AND

Ristocetin AND automated” Found a cross-sectional study

– Abstract included details of interest and study objectives paralleled those of scenario

Critical Appraisal

Objectives

Are the objectives of the study clearly stated?– Yes

Design

Is the study design suitable for the objectives?– Yes– The study was a prospective cross-sectional

study, which is the best study design for assessing diagnostic tests. • Advantages: simple, ethically safe, relatively cheap• Disadvantages: establishes association, recall bias,

confounders, unequal group sizes

Design

Who/what was studied?

Design

Was this the right sample to answer the objectives?– Yes– The authors selected a sample that avoids spectrum

bias• All were suspected to have vWD; none were obviously

diseased or obviously healthy

– The patients were recruited consecutively indicating that selection bias is less likely.

– Several blood types were represented. However, only two types of vWD were represented, indicating the full spectrum of vWD patients were not represented.

Design

Is the study large enough to achieve its objectives? Have sample size estimates been performed?– Uncertain

• There are no indications that a power analysis was performed.

• The sample size was relatively small (54 samples had complete data), but this is acceptable because vWD is relatively rare.

• The authors admit that studies with more patients need to be done.

Design

Were all subjects accounted for?– Yes– They did not provide all the individual sample data but

the tables all add up.

Design

Were all appropriate outcomes considered?– Yes– The authors considered all possible test result

combinations Has ethical approval been obtained if

appropriate?

Design

Was an independent blinded gold standard test applied to all subjects?– The gold standard test, ristocetin cofactor

assay, was performed on all subjects. – It does not seem to have been independent

• The authors performed both the gold standard and the test assay

– It is unclear if the researchers were blinded, but considering the objectivity of the measures being compared, this may not be very critical.

Measurement

Is it clear what was measured, how it was measured, and what the outcomes were?– Agglutination of latex reagent at 405 nm– Decrease in light transmittance– Reference curves were utilized– MDA-180 & ACL TOP coagulation analyzers

were utilized

Measurement

Are the measurements valid?

Measurement

Are the measurements reliable?– No conspicuous discrepancies exist amongst

samples and equipment

Measurement

Are the measurements reproducible?– Not well described– They are likely to be reproduced due to quality

control tests from the manufacturers, no significant differences between coagulation analyzers, and standard manufacturers’ protocol (and dilutions) applied to all assays

Results

Are the basic data adequately described?– Yes– Percentage agglutinations of each sample are

clearly grouped and explained as to whether or not vWD is associated and, if so, which type

Results Are the results presented clearly,

objectively, and in sufficient detail to enable readers to make their own judgment?– Yes

Results

Are the results internally consistent, i.e. do the numbers add up properly?– Yes– Numerators and denominators included for

most percentage calculations – Table 2 (previous slide) provided to show

exactly how all cases were classified

Analysis

Are the data suitable for analysis?– Yes– Sensitivity and specificity desired for diagnostic

study analyses

Analysis

Are the methods appropriate to the data?– Yes– For the type of

diagnostic analysis needed, the agglutination methods are appropriate

Analysis

Are any statistics correctly performed and interpreted?– Hard to tell because no clear chart or individual

patient data is provided– However, from the Table 2, we can calculate:

• Specificity= 36/42= 86%• Sensitivity= 12/12= 100%

– Positive predictive and negative predictive values provided because the true spectrum of patient population is not represented

Discussion

Are the results discussed in relation to existing knowledge and on the subject and study objectives?– Yes – The results explicitly mention the current gold standard of

testing and the relation of the new method. – They briefly mention the relation of vWD and blood types, Factor

VIII activity and PTT as well. – Furthermore, the results are provided as evidence to support the

study objective, which was to analyze a new, quicker, easier test to screen for (and ultimately diagnose) vWD that could mitigate the reliance upon more complex tests, such as the Ristocetin Cofactor tests, to potentially save time and money.

Discussion

Is the discussion biased?– The discussion does not appear to be biased,

but it is unknown if the researchers have a conflict of interest

– They clearly favored the implementation of the new test, however, the data supported their claims of quality.

Interpretation

Are the authors’ conclusions justified by the data?– Yes– The data had 100% sensitivity and over 85%

specificity, which supports the claim that the new test is a viable screening alternative.

– However, the study was small, there was still sufficient heterogeneity, and all types of results were found, including normal.

Interpretation

What level of evidence has this paper presented (using CEBM levels)?– Level 1b or 1c

• Study was a “validating” (not exploratory) study• Had good reference standard application• Absolute SnNout (sensitivity=100%)

– Grade A recommendation • consistent with a level 1 study

OCEBM Levels of Evidence Working Group. "The Oxford 2009 Levels of Evidence". Oxford Centre for Evidence-Based Medicine. http://www.cebm.net/?o=1025

Interpretation

Does this paper help me answer my problem?– Yes, it does, at least preliminarily. – I can utilize this new test with my patient (as long as it

is cost-efficient) to rule out vWD. However, if the results are abnormal, I will have to retest utilizing the gold standard of the Ristocetin Cofactor test to be certain.

How do you rate this paper? 8– It seems to provide useful information, but with a

limited patient population and data set, I cannot be certain of the validity of the results

Case Review

A 21 year old woman presents to your office for evaluation of abnormal menstrual periods. Her periods last from 7 to 9 days requiring up to 10 pads a day (excessive). On further questioning, she admits to easy bruising and nosebleeds. Family history reveals that her mother has had similar problems.

You suspect von Willebrand disease (vWD). vWD is the most common inherited coagulation abnormality.

Your laboratory informs you of a new automated assay for vW factor (vWF) activity. Since you have not heard about it, you decide to check what the published literature says about this new test and how does it compare to the established ristocetin cofactor method that you used to order to confirm the diagnosis of vWD.

Diagnosis of von Willebrand Disease

Three main criteria:– Past medical history of excessive bleeding

• Periods last from 7 to 9 days, requiring up to 10 pads per day

• Easy bruising• Nosebleeds

– Family history of excessive bleeding• Mother has had similar problems

– Laboratory evaluation of vWF

Diagnosis of von Willebrand Disease

Laboratory evaluation of vWF– Follow Figure 2

to determine procedure for ordering tests:

Implementation

Can any necessary change be implanted in practice?– Yes– As long as the new test is readily available and

cost-effective, it would likely be a favored alternative

Implementation

What aids to implementation exist?– Technology, marketing, and coding. Since technology

is so advanced, it should be relatively easy to order the required supplies and have the tests available “in-house” in a short period of time.

– With marketing, more people could be informed about the new, “easy” test and opt to have it done, especially if it is faster, cheaper, and easier than the old method.

– Depending on the medical coding being used in each clinic or hospital, a new code could be created to bill the patient or the patient’s insurance for the test.

Implementation

What barriers to implementation exist?– Insurance companies might be reluctant to pay for new

technology, especially if it implicates further disease states that it will be responsible for covering.

– Some physicians and/or facilities may not want to try something new, especially if they are used to the “old ways” of doing things.

– Depending on how much time and money is really saved by the new method, insurance companies may not want to cover it, especially if the patients end up having a second test done as well.

Implementation

Are my patients the same as the patients tested?– Yes, approximately. – The mean age was 37 (range 2-89 years), and

my patient is 21, so she falls within the range of patients tested.

Implementation

Will the test improve diagnosis in my patients?– It may not improve the diagnosis per se,

however, it may make the process easier and completed more quickly and easily, which could potentially correlate to savings of money and time.

– Therefore, if I had several patients who needed to be tested, it might improve their diagnosis in that they get it more quickly and thus receive proper treatment more quickly as well.

References

1. Salem, RO, Van Cott, EM. A new automated screening assay for the diagnosis of von Willebrand disease. Am J Clin Pathol 2007; 127:730-735.

2. De Vleeschauwer, A, Devreese, K. Comparison of a new automated von Willebrand factor activity assay with an aggregation von Willebrand ristocetin cofactor activity assay for the diagnosis of von Willebrand disease. Blood Coagul Fibrinolysis 2006; 17:353-358.