Laboratory tests in pregnancy Flashcards
1
Q
Amniotic fluid bilirubin (change OD 450)
- clinical significance
- specimen characteristics
A
- The concentration of unconjugated bilirubin in amniotic fluid is a reflection of the severity of fetal hemolysis
- When assessed by scanning spectrophotometry, the maximal absorbance of bilirubin is at 450 nm
- Amniotic fluid specimen
- Should be obtained with minimal blood contamination because the tail of the absorbance peak of oxyhemoglobin at 410 nm can affect the magnitude of the peak at 450 nm
- Should be protected from light (a brown plastic tube usually provided in the amniocentesis tray) because bilirubin is rapidly degraded by it
2
Q
Amniotic fluid bilirubin measurement and interpretation
A
- absorbances measured at intervals from 340-560 nm
- on semilog paper, the absorbances are plotted against wavelength and a straight line drawn from the point at 350 nm to the point at 550 nm (line reflects the theoretical plot if there were no pigments in the fluid)
- The difference in optical density between the line and the actual absorbance at 450 nm is the “deltaOD 450”, which reflects the bilirubin concentration
- Interpretation:
- delta OD 450 is plotted against the estimated gestational age (EGA) on a Liley chart or similar nomogram to determine the degree of fetal hemolysis
3
Q
Human chorionic gonadotropin
- produced by
- structure
- False positive hCG
True positive, low level hCG
A
-
Produced by
- normal placenta
- tumors
- small quantity may be produced by the pituitary gland, particularly during menopause
- Heterodimer composed of an alpha and beta chain; alpha chain identical to that found in TSH, FSH, and LH, while the beta subunit is unique
-
False positive hCG
- most often results from heterophile antibody interference
-
True positive, low level hCG
- exclude heterophile antibody, pregnancy, or active gestational trophoblastic disease
- sometimes found to have quiescent trophoblastic disease
- pituitary production of hCG may be found in
- perimenopausal women
- pituitary tumor
4
Q
hCG in normal gestation
A
- becomes detectable 6-8 days following conception; levels are 10-15 mIU/mL
- doubles roughly every 48 hours until ~10 weeks
- peaks near end of 1st trimester
- then decreases gradually and by early 2nd trimester plateaus
- hCG may be higher than usual in pregnancies with multiple gestation, polyhydramnios, eclampsia, and erythroblastosis fetalis
- after delivery hCG remains detectable for 2 weeks
5
Q
hCG in ectopic pregnancy
A
- abnormal hCG dynamic (fails to rise at least 66% in 48 hours) suggests an abnormal pregnancy (ectopic or nonviable intrauterine pregnancy)
- however, normal rate of rise can be seen in 20% of ectopics, and an abnormal rate of rise can be seen in 20% of normal intrauterine pregnancies
- further elucidation relies on US demonstration of an intrauterine gestational sac and/or serum progesterone level
- after removal of ectopic pregnancy, hCG remains detectable for several weeks
6
Q
hCG in gestational trophoblastic disease (GTD)
A
- women with GTD usually produce a greater amount of hCG than normal gestations; hCG levels are generally higher in complete moles than partial moles
- After molar pregnancy evacuation, hCG levels monitored weekly until undetectable for 3 weeks, then measured monthly for 1 year
- after evacuation of an uncomplicated molar pregnancy, hCG remains detectable for up to 10 weeks; if the hCG plateaus or rises, then persistent GTD is suspected
7
Q
Common laboratory values in pregnancy
A
8
Q
Prenatal screening for trisomy epidemiology
A
- Original screening test for tri 21 and tri 18 was maternal age
- Risk of having affected neonate is 1/700 overall
- Risk increases to 1 in 270 for women older than 34
- Most trisomies occur in women < 35
- All women, regardless of age should be offered screening
9
Q
Pregnancy screening approaches for trisomy and NTD
A
- The quad screen
- maternal serum hCG
- AFP
- unconjugated estriol (uE)
- dimeric inhibin A
- drawn in 2nd trimester (18 weeks)
- Risk is calculated based on these analytes + maternal age
- Sensitivity for Downs is 78%
- The first trimester test
- performed at 10-13 weeks
- hCG
- pregnancy associated plasma protein A (PAPP-A)
- US assessment of nuchal fold translucency thickness
- When combined with maternal age the overall sensitivity is 83%
-
Serum integrated screen
- combines first and second trimester testing
- when combined with maternal age and nuchal fold thickness sensitivity is 88%
-
Sequential screen
- risk is initially reported based upon the 1st trimester results if (>1:25)
- If the first trimester results do not indicate high risk, then the risk is reported based upon the integrated results
10
Q
Trisomy risk calculation
A
- serum markers vary with gestation age, so they are expressed as multiple of median (MoM)
- Risk calculation
- Use risk based on age at delivery with epidemiology
- Then a likelihood ratio for each analyte’s MoM is determined
- These are multipled by the age associated risk to arrive at an adjusted risk
- Adjustments are made for maternal weight, race, and diabetes
- Each lab applies a risk cutoff to report the screen as positive or negative; cutoffs vary, but usually use the risk of a 35 year old woman
11
Q
Serum marker levels in Downs, Tri 18, and neural tube defects
A
- Trisomy 21
- low AFP
- low uE
- raised hCG
- raised DIA
- Trisomy 18
- low AFP
- low hCG
- low uE
- Neural tube defect
- raised AFP
- normal hCG
- low uE
12
Q
Assessing the risk of preterm birth
A
- Serum estradiol and salivary estriol are both increased before onset of labor, but clinical accuracy is suboptimal
- Screening for bacterial vaginosis has value, associated with increased risk of preterm birth
- Fetal fibronectin
- found normally at the placental fetomaternal interface
- absence of fetal fibronectin in cervicovaginal fluid has high NPV and excludes impending preterm birth
- A positive result suggests onset of preterm labor, but overall PPV is low
- Transvaginal cervical US to assess cervical length has diagnostic accuracy similar to fetal fibronectin
13
Q
Determination of fetal lung maturity
A
- Fetal lung maturity is achieved at 37 weeks
- stress of complicated pregnancy produces excess corticosteroid which accelerates FLM
- maternal diabetes delays FLM
- Should assess FLM in 32-38 week gestation
- Uncontaminated amniotic fluid obtained by amniocentesis is optimal specimen; vaginal pool specimens from women with PROM should be avoided as should specimens with blood or meconium
- Most tests for FLM are better at predicting maturity than immaturity
- A mature result by any method is reliable and result below maturity cutoff should be confirmed by a second test by another technique
14
Q
Lecithin/Sphingomyelin ratio
A
- ratio of >= 2.5:1 indicates FLM
- in diabetic moms the above ratio is less predictive of FLM; phosphatidylglycerol is more reliable in this setting
- Meconium falsely decrease L:S
- Blood normalizes the L:S ratio to 1.5
15
Q
Phosphatidylglycerol concentration
A
- PG is first detected ~35-36 weeks
- Presence is indicative of FLM
- Blood and meconium do not interfere
- PG is a late marker of FLM which limits its utility in preterm babies
