beta-chorionic gonadotropin (hCG) in serum/plasma

Introduction

Also see ARUP consult

Reference interval

• Concentration of B-hCG in normal nonpregnant individual: <4mIU/mL.
• Concentration of B-hCG during pregnancy rises rapidly in the first trimester & declines slowly throughout the remainder of the pregnancy.
• A sharply reduced or falling serum B-hCG level may indicate an abnormal pregnancy, & additional clinical evaluation & follow-up may be appropriate.
• Approximate HCG ranges during normal pregnancy are summarized below

Gestational Age	Range hCG (mIU/mL)
0-1 Week	0- 50
1-2 Week	40- 300
2-3 Week	100- 1,000
3-4 Week	500- 6,000
5 Weeks	217-7138
1-2 Months	5,000- 200,000
2-3 Months	10,000- 100,000
2nd Trimester	3000- 50,000
3rd Trimester	1000- 50,000

Principle

The IMx B-hCG assay is based in the Microparticle Enzyme Immunoassay (MEIA) technology.

The reagents are added to the reaction cell in the following sequence:

• The probe/electrode assembly delivers the sample,IMx Total B-hCG Specimen Diluent, & Anti-BhCG Coated Microparticles to the incubation well of the reaction cell.
• The B-hCG binds to both the enzyme-labeled antibody & the Anti-B-hCG Coated Microparticles forming an antibody-antigen complex.
• An aliquot of the reaction mixture containing the antibody- antigen complex bound to the microparticles is transferred to the glass fiber matrix. The microparticles bind irreversibly to the glass fiber matrix.
• The matrix is washed to remove unbound materials.
• The substrate, 4-Methyllumbelliferyl Phosphate, is added to matrix & the fluorescent product is measured by the MEIA optical assembly.

The Stratus b-hCG Fluorometric Enzyme Immunoassay uses two monoclonal antibodies specific for distinct antigenic sites on the b subunit of the hCG molecule, resulting in an assay highly specific for b-hcg as either free subunit or as intact hCG. The Stratus b-hCG Fluorometric Enzyme Immunoassay is a rapid & sensitive automated procedure for the quantitative determination of total b human chorionic gonadotropin in serum & plasma. The bhCG assay range is from 2 mIU/mL to 1000 mIU/mL. Patient samples containing b-hCG levels greater than 1000 mIU/mL may be quantified by dilution in High Sample Diluent Buffer.

The Stratus bhCG assay is based upon a two-site sandwich immunoassay methodology. In this procedure, the clinical sample is pipetted onto the center portion of a square piece of glass fiber paper where it reacts with immobilized monoclonal anti-b-hCG antibody directed toward a unique antigenic site on the b subunit of hCG. After a short incubation, a conjugate, consisting of enzyme-labeled monoclonal anti-b-hCG antibody directed against a different antigenic site on the b subunit of hCG, is pipetted into the reaction zone. During the second incubation period, the labeled antibody reacts with the antibody-bound b-hCG forming an antibody/antigen/labeled antibody sandwich. The unbounded labeled antibody mixture is later eluted from the field of view of the Stratus Fluorometric Analyzer by applying a substrate wash solution to the center of the reaction zone & removing the unbound fraction by radial elution. By including substrate for the enzyme within the wash solution, initiation of enzyme activity occur simultaneously with the wash. The enzymatic rate of the bound fraction, which varies directly with the concentration of b-hCG present in the sample, can then be measured by an optical system that monitors the reaction rate via front-surface fluorescence. All data analysis functions are performed by the micro-processor within the analyzer.

Clinical significance

• hCG is initially secreted by the trophoblastic cells of the placenta shortly after implantation of the fertilized ovum into the uterine wall
• the rapid rise in hCG serum levels after conception makes for early confirmation & monitoring of pregnancy

• physiologically, hCG appears to maintain the corpus luteum, thereby allowing synthesis of progesterone & estrogens that support the endometrium
• as uncomplicated pregnancies progress, the placenta assumes the production of these hormones
• the serum hCG levels increase to a peak concentration, then decrease & plateau
• hCG circulates as the intact molecule in the serum of normal women who have an uncomplicated pregnancy
• the subunits are cleaved rapidly & cleared by the kidney.

• radioimmunoassay detects beta hCG at 5 mIU/mL 7-9 days after conception (99-100% sensitive)
• hCG is still detectable 9-35 days following a spontaneous abortion & 16-60 days after an induced abortion

Increases

• pregnancy, including ectopic pregnancy
• hydatidiform mole
• choriocarcinoma
• germ cell tumors

Specimen

• Serum specimens & plasma (heparin only) may be used.
• If assay is performed within 24 hours after collection, the specimen should be stored in the refrigerator at 2-8 C. If the testing will be delayed more than 24 hours, the specimen should be frozen. Mix thoroughly after thawing to ensure consistency in the results. Avoid repeated freezing & thawing.
• Specimens showing particulate matter, erythrocytes, or turbidity should be centrifuged before testing.

SAMPLE VOLUME:

• IMX: 150 uL of specimen is the minimum volume required to perform the assay
• Stratus: minimum sample size: 1 mL; with an optimum size of 1.5 mL

More general terms

• beta-chorionic gonadotropin (hCG) in body fluid
• pregnancy test

More specific terms

• choriogonadotropin.beta subunit free in serum/plasma

Additional terms

• beta-chorionic gonadotropin (hCG) free in serum/plasma
• chorionic gonadotropin [cG]-beta chain (CG-beta, beta-hcG, CGB)
• chorionic gonadotropin; choriogonadotropin (APL, hCG)

Component of

• choriogonadotropin intact + beta subunit in serum/plasma

Patient information

beta-chorionic gonadotropin (hCG) in serum/plasma patient information