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Test Code MULT Zygosity Testing (Multiple Births)

Useful For

Determining genetic risk for an individual whose twin or triplet is affected with a genetic disorder for which a specific genetic test is not available (or such testing is uninformative)


Assessment of risks prenatally when 1 fetus of multiples is known to be affected by a specific disorder


Organ or bone marrow transplantation compatibility testing


Familial or parental interest

Genetics Test Information

DNA from twins and their parents is used to determine if the twins are identical (monozygotic) or fraternal (dizygotic).

Reflex Tests

Test ID Reporting Name Available Separately Always Performed
CULFB Fibroblast Culture for Genetic Test Yes No
CULAF Amniotic Fluid Culture/Genetic Test Yes No
_STR1 Comp Analysis using STR (Bill only) No No
_STR2 Add'l comp analysis w/STR (Bill Only) No No

Testing Algorithm

For prenatal specimens only: If amniotic fluid (nonconfluent cultured cells) is received, amniotic fluid culture/genetic test will be added and charged separately. If chorionic villus specimen (nonconfluent cultured cells) is received, fibroblast culture for genetic test will be added and charged separately.

Method Name

Polymerase chain reaction (PCR)/microsatellite markers are used to provide a genotype for multiples and their parents to determine if siblings are monozygotic twins.

Reporting Name

Zygosity Testing (Multiple Births)

Specimen Type


Shipping Instructions

Specimen preferred to arrive within 96 hours of draw.

Necessary Information

A blood specimen from both parents, in addition to a specimen from each multiple, is required. Each specimen will be charged separately.

Specimen Required

Patient Preparation: A previous bone marrow transplant from an allogenic donor will interfere with testing. Call Mayo Medical Laboratories for instructions for testing patients who have received a bone marrow transplant.


Submit only 1 of the following specimens:


Specimen Type: Whole blood

Container/Tube: Lavender top (EDTA) or yellow top (ACD)

Specimen Volume: 3 mL

Collection Instructions:

1. Invert several times to mix blood.

2. Send specimen in original tube.

Specimen Stability Information: Ambient (preferred)/Refrigerated


Due to the complexity of prenatal testing, consultation with the laboratory is required for all prenatal testing. Prenatal specimens can be sent Monday through Thursday and must be received by 5 p.m. CST on Friday in order to be processed appropriately. All prenatal specimens must be accompanied by a maternal blood specimen. Order MATCC / Maternal Cell Contamination, Molecular Analysis on both the prenatal and the maternal specimens.


Specimen Type: Amniotic fluid

Container/Tube: Amniotic fluid container

Specimen Volume: 20 mL

Specimen Stability Information: Refrigerated (preferred)/Ambient


Specimen Type: Chorionic villi

Container/Tube: 15-mL tube containing 15 mL of transport media

Specimen Volume: 20 mg

Specimen Stability Information: Refrigerated



Specimen Type: Confluent cultured cells

Container/Tube: T-25 flask

Specimen Volume: 2 flasks

Collection Instructions: Submit confluent cultured cells from another laboratory.

Specimen Stability Information: Ambient (preferred)/Refrigerated

Specimen Minimum Volume

Blood: 0.5 mL
Amniotic Fluid: 10 mL
Chorionic Villi: 5 mg

Specimen Stability Information

Specimen Type Temperature Time
Varies Varies

Reject Due To

No specimen should be rejected.

Clinical Information

Approximately 30% of twins are monozygotic (identical), while 70% are dizygotic (nonidentical or fraternal). Monozygotic twins originate from a single egg, and by definition have identical DNA markers throughout their genomes. Dizygotic twins, on the other hand, inherit their genetic complement independently from each parent and are no more likely to have genetic material in common than are any other full siblings.


To date, literally thousands of polymorphic DNA markers have been identified. DNA markers are regions of DNA that display normal variability in the type or the number of nucleotide bases at a given location. One particular class of repetitive DNA that exhibits marked variability is microsatellites. With the use of such markers, it is possible to distinguish one individual from another because of differences detected at these polymorphic loci. Utilizing PCR followed by gel electrophoresis, the genotypes of a set of twins (triplets, etc.) are derived from the analysis of multiple markers. This genotype is compared to those of their parents to determine if the children are mono- or dizygotic. Any differences detected between siblings' microsatellite markers indicate dizygosity.


Many disorders are known to occur on a genetic basis though the genes have not been identified for all of them. If 1 member of a set of twins is diagnosed with a genetic disorder, determination of zygosity, in addition to other testing, may provide additional information regarding risk assessment of unaffected individuals. In addition, zygosity can be useful when evaluating for twin-twin transfusion syndrome during pregnancy or as part of a preorgan transplant workup for situations where one twin is donating an organ to another twin.

Reference Values

An interpretive report will be provided.


An interpretive report will be provided.


Test results should be interpreted in the context of clinical findings, family history, and other laboratory data. Errors in our interpretation of results may occur if information given is inaccurate or incomplete.


This test will detect nonpaternity.


Availability of a specimen from all family members (multiples and parents) provides the most accurate results. If parental blood is not available, markers may not be informative.

Clinical Reference

1. Appleman Z, Manor M, Magal N, et al: Prenatal diagnosis of twin zygosity by DNA "fingerprint" analysis. Prenat Diagn 1994;14(4):307-309

2. Neitzel H, Digweed M, Nurnberg P, et al: Routine applications of DNA fingerprinting with the oligonucleotide probe (CAC)5/(GTG)5. Clin Genet 1991;39(2):97-103

Method Description

PCR-based assays that recognize highly variable regions of human DNA are used to provide a genotype for multiples and their parents. The number of markers (microsatellites) used is determined on a case-by-case basis to ensure >99.9% predictive value. Generally, between 6 and 11 markers are used. Calculation of zygosity probability is made using Bayesian analysis. (Neitzel H, Digweed M, Nurnberg P, et al: Routine applications of DNA fingerprinting with the oligonucleotide probe [CAC]5/[GTG]5.Clin Genet 1991;39[2]:97-103)

Day(s) and Time(s) Performed

Monday; 10 a.m.

Analytic Time

5 days

Specimen Retention Time

Whole Blood: 2 weeks (if available) Extracted DNA: 3 months

Performing Laboratory

Mayo Medical Laboratories in Rochester

Test Classification

This test was developed and its performance characteristics determined by Mayo Clinic in a manner consistent with CLIA requirements. This test has not been cleared or approved by the U.S. Food and Drug Administration.

CPT Code Information

81265 Comparative analysis using Short Tandem Repeat (STR) markers; patient and comparative specimen (eg, pre-transplant recipient and donor germline testing, post-transplant non-hematopoietic recipient germline [eg, buccal swab or other germline tissue sample] and donor testing, twin zygosity testing or maternal cell contamination of fetal cells

 Fibroblast Culture for Genetic Test

88233-Tissue culture, skin or solid tissue biopsy (if appropriate)

88240-Cryopreservation (if appropriate)


Amniotic Fluid Culture/Genetic Test

88235-Tissue culture for amniotic fluid (if appropriate)

88240-Cryopreservation (if appropriate)


Added as needed:

81266 each additional specimen (eg additional cord blood donor, additional fetal samples from different cultures, or additional zygosity in multiple birth pregnancies)

LOINC Code Information

Test ID Test Order Name Order LOINC Value
MULT Zygosity Testing (Multiple Births) In Process


Result ID Test Result Name Result LOINC Value
53322 Result Summary 50397-9
53323 Result 69548-6
53324 Interpretation 69965-2
53349 Reason for Referral 42349-1
53325 Specimen 31208-2
53326 Source 31208-2
53327 Method 49549-9
53328 Released By 18771-6


1. Molecular Genetics: Congenital Inherited Diseases Patient Information (T521) in Special Instructions

2. New York Clients-Informed consent is required. Please document on the request form or electronic order that a copy is on file. An Informed Consent for Genetic Testing (T576) is available in Special Instructions.