Lecture 4 Flashcards
Embryo Development
Ovulation varies from person to person, fertilization takes 24 hours after ovulation in the distil end of fallopian tube
The zygote begins to divide and morphs into blastocyst as it continues its journey down the fallopian tube
The embryo enters the uterine cavity and implants itself in the uterine wall or endometrium (4-5 days), forming a sac or placenta
The embryo and placenta starts producing HCG on implantation
Week 8, embryonic stage ends and is referred to as a fetus
Pregnancy Testing
Rely on the presence of a hormone – human chorionic gonadotropin (hCG)
Sometimes referred to as uterine chorionic gonadotropin (uCG)
hCG is produced by the placenta and developing embryo
HCG is detectable 1 week in serum and 10 -14 days in urine
HCG
glycoprotein and contains two sub-units – Alpha and Beta
HCG doubles every 48/72 hours
Peak levels are reached about the 12th week of pregnancy
Declines about the third month, decreases to a level of 10,000 -20,000 IU/ml till end of pregnancy
Not detectable a few days after delivery
What does hCG do
maintains progesterone production that is critical for pregnancy
Progesterone enriches the uterus with a thick lining of blood vessels and capillaries necessary to sustain the growing fetus
Levels of hCG may be measured in the blood or urine
In Urine - referred to as pregnancy test (measuring Beta HCG)
In Blood – Beta human chorionic gonadotropin -
βhCG
Immunoassay Test – “Elisa or Sandwich” Method
Urine Pregnancy Test
Provide the rapid, qualitative detection of hCG
Monoclonal antibody on the membrane that binds specifically to BHCG and a second antibody that links to Alpha HCG.
When specimen is applied, it travels up the membrane – react according to the presence or absence of BhCG
A chromogenic substance undergoes a colour change in the presence of BHCG
Results are observed as a colour change or as + or - on the reaction kit
Interpretation is based on the manufacturer instructions (within the kit)
Limitations of the Immunoassay Test – “Elisa or Sandwich” Method
Urine Pregnancy Test
A normal pregnancy cannot be distinguished from an ectopic pregnancy based on hCG levels alone.
A spontaneous miscarriage may cause confusion in interpreting the test results.
A negative result from a specimen collected from a woman in very early pregnancy may be due to an unusually low concentration of hCG. In such cases, the test should be repeated on a fresh specimen obtained approximately two days later.
A urine sample may be too diluted and thus may not contain a representative concentration of hCG. If a negative result is obtained with a urine specimen and pregnancy is still suspected, obtain a first morning midstream urine specimen and re-test.
Guidelines –Urine specimen (Pregnancy Test)
Use clean, sterile, disposable urine containers
Use a first-voided morning specimen – contain the highest concentration of hCG
A specific gravity < 1.010 is considered too dilute for pregnancy testing – may lead to a false negative result
Specimen should be at room temperature and mixed, before performing the test\
Serum Pregnancy Test
More sensitive than a urine test
Can detect pregnancy approximately the seventh day after fertilization
Allows quantitative detection of βhCG
Can be used to monitor and diagnose abnormalities in pregnancy & ectopic pregnancy
A sharp spike/increase in concentration is indicative of a problem in the pregnancy
Quantitative BHCG
Doubling of BHCG indicates the fetal viability
Facilitates the detection of :
Suspected ectopic pregnancy (BHCG not doubling every 48 hours)
Viability of the fetus and threatened abortions
Suspected fetal anomalies e.g high levels of BHCG found in twin pregnancies and Down’s syndrome (Trisomy 21)
Suspected Chorionic Carcinoma
Germ cell tumor (GCT)
Testing for hCG may also be done when diagnosing or monitoring germ cell tumors and gestational trophoblastic disease
Can be cancerous or non-cancerous tumors
Normally occur inside the gonads (ovary and testes [testicular cancer])
Cause very elevated hCG levels (false positive)
Gestational trophoblastic disease (GTD)
A group of rare tumors that involve abnormal growth of cells inside the uterus
GTD does not develop from cells of the uterus like cervical cancer or endometrial (uterine lining) cancer
These tumors start in the cells that would normally develop into the placenta during pregnancy
Cause very elevated hCG levels (false positive)
Fecal Occult Blood – Colorectal Cancer
Colorectal cancer is one of the most common cancer diagnosed in Canada
Second leading cause of cancer deaths
Polps can form along the colon rectal passage. Blood vessels on these polyps are fragile and are easily damaged by the passage of feces. The blood vessel release a small amount of blood
Positive occult blood can mean there is a possible cancer, polyps, hemorrhoids, or bleed due to an ulcer diverticulosis or inflammatory bowel disease
If positive, a colonoscopy is done
Why is Fecal Occult Blood done?
Fecal screening to detect small invisible quantities of blood (hemoglobin) in stool
Used to detect bleeding in the gastrointestinal tract
Main purpose of test is to screen for colon cancer
May also indicate colorectal cancer, polyps, ulcers or other lesions
How does the test of Fecal Occult Blood done?
Test procedures use a paper impregnated with a resin called guaiac which produces a blue colour when oxidized in the presence of hydrogen peroxidase (H2O2)
Heme contains pseudoperoxidase which converts guaiac to the blue colour
When H2O2 is dropped on the guaiac paper containing stool, and if blood is present, the peroxidase from the heme release oxygen and converts the guaiac to a blue colour
Results for Fecal Occult Blood
Three separate stool samples taken on 3 consecutive days should be submitted for testing
Patient should not eat red meat for 3 days prior to the test and continuing throughout the test period
Red meat contain hemoglobin and myoglobin that can cause false positive results
Large doses of vitamin C or grapefruit – false negative
Contamination with menstrual blood – false positive
Some drugs (iron) – may cause false positives
Some food such as turnips, horseradish, cauliflower, cabbage, carrots can cause a false positive result
Patient instructed to avoid specimen contamination with H2O, urine, blood from bleeding - menstrual cycles (and up to 3 days after)
Protect test card from heat, light or volatile liquids
