Anti-Acetylcholine Receptor Antibody Diagnostic Testing (AChRAb)
Myasthenia Gravis
Myasthenia Gravis (MG) is an autoimmune disease of the neuromuscular junction (NMJ) characterized primarily by weakness and fatigability of voluntary muscles. There exists three types of acquired MG: ocular (OMG), bulbar predominant (BMG), and generalized (GMG). The primary mechanism for the clinical features of MG is known to be an antibody-mediated autoimmune attack against nicotinic acetylcholine receptors (AChR) at the NMJ. These autoantibodies disrupt neuromuscular transmission by either blocking acetylcholine binding sites or reducing the number of receptors at the muscle membrane. Autoantibodies to AChRs are detected by a radioimmunoprecipitation assay in 80-90% of sera from patients presenting clinical features MG. The high frequency of anti-AChR Abs in MG patients and their specificity makes their detection a reliable diagnostic test1.
Anti-AChRAb Testing at the Neuro-Immunology Laboratory
Assaying for anti-AChRAbs using a radioimmunoprecipitation technique for diagnostic purposes of MG has been offered at the Neuro-Immunology Laboratory since 1984. Over 20,500 serum samples from across Canada have been received for anti-AChRAb testing by the lab between Jan 1984 and Feb 2011. The lab has been accredited by the Diagnostic Accreditation Program of British Columbia and the test is insured across Canada in application of the Reciprocal Agreements between provinces. They are charged to the Medical Services Plan of BC and reimbursed to BC by the Health Care Plan of each province, except Quebec for which we have to bill the referring labs.
The radioimmunoprecipitation assay used for AChRAb testing is highly sensitive and specific for the AChRAb-seropositive form of MG. Measurement of anti-AChRAb titers aids in the diagnosis of MG and repeated testing can provide physicians with valuable information regarding course of the disease during treatment.
This highly sensitive and specific test is aimed at detecting anti-acetylcholine receptor antibodies of all levels (high and low). We use a radio-immunoprecipitation assay with a double antibody precipitation technique. The antigen we use is the Kronus antigen, a genetically engineered acetylcholine receptor adapted to include both adult and fetal acetylcholine receptor subtypes. A comparison is made of complex precipitated by test sera versus control sera. The test requires 5 mL of serum per sample (see technical information below). Clinical information is required.
The Neuro-Immunology Lab offers 2 tests: QUALITATIVE & QUANTITATIVE
i) Qualitative - MSP# 91020, $101.00 CAD
Results are expressed in the following way:
ABSENT – Samples found to be negative for AChR Abs (this corresponds to <0.1nmol/L, i.e. <0.1nmol of alpha-Bungarotoxin precipitated by 1L of serum).
BORDERLINE – This corresponds to levels of precipitation between the mean+2 Standard Deviation (SD) and mean +3SD of a group of healthy controls, but that is less than our low positive control. When we encounter a borderline result, the assay is repeated before sending out results, and a request is made for a second sample to be sent to us after a few weeks.
All samples found to be POSITIVE for AChR Abs following a qualitative assay are repeated using the quantitative assay and results are sent out quantitatively (see below).
ii) Quantitative – MSP# 91021, $165.98 CAD
All samples found to be positive by the qualitative assay are repeated using the quantitative assay. We re-assay the previous sample, if available, to permit comparison of titres.
POSITIVE results correspond to levels of precipitation greater than mean+3SD of a group of healthy controls. Results are given as a titer in units of nmol/L (nmol of alpha-Bungarotoxin
precipitated by 1L of serum).
Technical Information
Serum samples should be collected in SST tubes, clotted, centrifuged and
transferred to cryovials.
Five (5) ml of serum should be shipped according to IATA, ICAO and Canadian transportation goods regulation. Samples do not need to be frozen. Please courier your samples together with
a completely filled out requisition (see below). The clinical information should be filled out by the referring physician (whom we will call or write in case of discrepant results).
The test is run on a weekly basis. It can also be run on an emergency basis with a turn-around time of one working day once the sample is received. Courier your sample early and contact us either by:
E-mail: neuroimm.lab@ubc.ca
Lab Phone: (604) 822-7175
Office Phone: (604) 822-7696
Fax: (604) 822-0758
Results will be faxed if requested (please include a fax number on the requisition).
Ship Samples To:
Neuro-Immunology Laboratory
University of British Columbia Hospital
Room S-157, 2211 Wesbrook Mall
Vancouver, B.C. V6T 2B5
Canada
Freezing and Saving of Samples
The Myasthenia Gravis Association of British Columbia has provided us with two freezers to save patient samples. All samples received are frozen and saved systematically to permit further reference and conducting comparative assays. When we receive a sample, we check if a sample from a previous date is available (i.e. if the patient had previously been tested for AChR Abs at our facility). In this case, we re-assay the previous sample along with the new one in the same assay. This allows us to increase the reproducibility of the test and provide for comparison of titers over time for qualitative samples.
Requisitions (PDF Files):
Acetylcholine Receptor Antibody Requisition (BC Only)
Neuro-Immunology Lab Requisition (CANADA except BC)
Neuro-Immunology Lab Requisition (USA)
REFERENCES:
1Oger J, Kaufman R, Berry K. Acetylcholine receptor antibodies in myasthenia gravis: use of a qualitative assay for diagnostic purposes. Can J Neurol Sci 1987;14:297-302.