Kleihauer-Betke test 3332961 219735135 2008-06-16T17:25:00Z Forgetful5434 7291804 re-jigged the entire intro, moving most of it to a new section, and got rid of a massive repeat of a paragraph The '''Kleihauer-Betke ("KB") test''', '''Kleihauer-Betke ("KB") stain''' or '''Kleihauer test''', is a [[blood test]] used to measure the amount of [[fetal hemoglobin]] transferred from a [[fetus]] to a mother's [[bloodstream]]<ref name="pmid17191260"> {{cite journal |author=Katiyar R, Kriplani A, Agarwal N, Bhatla N, Kabra M |title=Detection of fetomaternal hemorrhage following chorionic villus sampling by Kleihauer Betke test and rise in maternal serum alpha feto protein |journal=Prenat. Diagn. |volume=27 |issue=2 |pages=139–42 |year=2007 |pmid=17191260 |doi=10.1002/pd.1632 |accessdate=2007-08-17}}</ref>. It is usually performed on [[Rh factor|Rhesus-negative]] mothers to determine the required dose of [[Rho(D) immune globulin]] (RhIg) to inhibit formation of Rh [[antibody|antibodies]] in the mother and prevent [[Rh disease]] in future Rh-positive children. ==Details of the test== The KB test is the standard method of detecting [[fetal-maternal hemorrhage]] (FMH). It takes advantage of the differential resistance of fetal hemoglobin to [[acid]]. A standard [[blood smear]] is prepared from the mother's [[blood]], and exposed to an acid bath. This removes adult [[hemoglobin]], but not fetal hemoglobin, from the red blood cells. Subsequent staining makes fetal cells (containing fetal hemoglobin) appear rose-pink in color, while adult red blood cells are only seen as 'ghosts'. A large number of cells (over 5000) are counted under the [[microscope]] and a [[ratio]] of fetal to maternal cells generated. In those with positive tests, follow up testing at a [[postpartum]] check should be done to rule out the possibility of a [[false positive]]. This could be caused by a process in the mother which causes persistent elevation of fetal hemoglobin, e.g. [[sickle cell trait]]. Comparison with other more expensive or technologically advanced methods such as [[flow cytometry]] has shown that the KB stain, like the more advanced methods, is sensitive in its detection of FMH.<ref name="pmid17191260" /> Background counting errors can result in [[estimate]]s of as much as 5 [[litre|ml]] fetal [[blood loss]] when there actually is no such blood loss, but standard methods available in most [[laboratory|laboratories]] admit an extremely low probability of the return of a false positive when more severe FMH has taken place. ==Estimating the Severity of FMH== To determine if a positive test for FMH should be considered as the likely cause of fetal [[death]], the [[percent]] of total fetal blood volume lost should be calculated, making appropriate adjustments based on the following known relationships: ** the size of a fetal red blood cell is 1.22 times that of an adult red blood cell; ** the KB stain is known to have a [[mean]] success rate of 92% in detecting fetal red blood cells; ** in a woman at or near term in her pregnancy, the mean volume of maternal red blood cells is approximately 1800 ml; ** the mean fetal [[hematocrit]] is 50%; and ** at [[stillbirth]], the mean fetal blood volume is <math>150 \frac{ml}{kg}</math> These [[constraint]]s can then be applied to yield the [[formula]] <center><math>PFB = \frac{(3200)(FC)}{(FW)(MC)}</math></center> where ** <math>PFB</math> is the [[percent]]age of fetal blood lost; ** <math>FC</math> is the observed number of fetal red blood cells; ** <math>MC</math> is the observed number of maternal red blood cells (N.B. we have that <math>MC = TC - FC</math>, where <math>TC</math> is the total observed number of red blood cells, both maternal and fetal); ** <math>FW</math> is the stillbirth weight of the fetus in [[kilograms]]. ==An Example of How This Formula is Used in Resolving a Stillbirth== Suppose that a KB stain is performed and <math>TC = 5000</math> total red blood cells are observed, <math>FC = 200</math> of which are found to be fetal red blood cells. Suppose further that the stillbirth weight of the fetus under consideration is <math>FW = 2.0 kg</math>. Then we would conclude that the total percentage of fetal blood lost is approximately <center><math>PFB = \frac{(3200)(FC)}{(FW)(MC)} = \frac{(3200)(200)}{(2.0)(4800)} = \frac{200}{3} = 66.667</math></center> to five [[significant digits]]. We would hence conclude that the fetus under consideration lost 66.667% (two thirds) of its blood via FMH. Generally, stillbirth is highly probable for any value of <math>PFB \geq 20</math>, particularly if the fetus abruptly loses this much blood; in this example, we would hence be likely to suspect FMH as the cause of the stillbirth. It is important to note, however, that such a [[diagnosis]] is still not completely conclusive; fetuses losing large quantities of blood over long periods of time are able to compensate for this slower blood loss; since the KB stain tells us nothing with regard to the level of acuity of FMH. This means that it is not possible to entirely [[correlate]] a positive KB stain and high <math>PFB</math> with a stillbirth, though in many cases, such as the stillbirth of [[Breanna Lynn Bartlett-Stewart]] in [[2000]], given other information, such as known [[hereditary]] [[Complication (medicine)|complication]]s of [[pregnancy]], extremely high positive [[correlation coefficient]]s <math>r \approx +1.000</math> between FMH and stillbirth have been observed. == Problems in the Detection of Fetal Red Blood Cells == Since fetal and maternal [[blood cell]]s have the same [[life expectancy]] in the maternal [[bloodstream]], it is possible to obtain informative results from a KB stain for a fair period of time after a stillbirth. However, if the mother and fetus are [[Blood type|ABO]] incompatible, it is more crucial to quickly perform the KB stain following a stillbirth, as the fetal red blood cells will be eliminated from the maternal bloodstream very quickly, causing the KB stain to underestimate the degree of FMH, if any. Lots of concern has been raised in the literature concerning false positives when sampling is done after delivery. In general this is not a problem. Delivery does result in higher frequency of detection of micro-hemorrhages but this should not confound interpretation of FMH as a possible cause of stillbirth. It is not necessary to draw the sample before induction, onset of labor, delivery, placental delivery etc. despite what some published literature purports. However, if caesarean section is to be used, failure to draw the sample prior to that will result in a 2% false positive rate. Finally, anything which causes persistence of fetal hemoglobin in maternal blood cells will make interpretation much trickier. Certain [[hemoglobinopathy|hemoglobinopathies]], the most common of which is sickle cell trait, do this. Overall, somewhere around 1-3% of the time this could result in false interpretation. == See also == *[[Apt test]] == References == <references/> [[Category:Blood tests]] [[Category:Obstetrics]] [[fr:Test de Kleihauer]]