| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
ORIGINAL RESEARCH |
From the Departments of *Obstetrics and Gynecology and Pathology, The Ohio State University College of Medicine and Public Health, Columbus, Ohio.
Address reprint requests to: Todd S. Albright, DO, 10193 Cape Ann Drive, Columbia, MD 21046; e-mail: live2bike{at}comcast.net.
 |
ABSTRACT |
|---|
TOP
 ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
METHODS: Amniotic fluid was collected from patients undergoing amniocentesis for obstetric indications between 31 and 40 weeks of gestation. A baseline TDxFLM II value was obtained and compared with amniotic fluid contaminated with 1%, 5%, and 10% meconium by weight.
RESULTS: Twenty-one samples were studied, and in every case the TDxFLM II value decreased once the meconium was added. There was no consistent rate of decrease that correlated with the percentage of meconium added.
CONCLUSION: Meconium contamination decreases the TDxFLM II value. A clinician who performs this test in the presence of meconium can be reassured that the contamination will not give an artificially elevated result. If the result is in the mature range, one can be confident that the result would only be higher if meconium were not present.
Tests for fetal lung maturity have been evolving over the past several decades. The lecithin/sphingomyelin ratio has been the most widely used test for many years and is the standard that modern tests have to meet. The lecithin/sphingomyelin ratio has an acceptable average sensitivity and specificity of 72% and 93%, respectively.1 However, the lecithin/sphingomyelin ratio is time consuming, taking 4–5 hours to perform. It also requires technical expertise and has both intra- and interlaboratory variations in results.2 In the face of contamination with blood or meconium, it is inadequate.3,4 For this reason, the lecithin/sphingomyelin ratio has been used in a laboratory cascade in which a phosphatidylglycerol assay is often performed. The phosphatidylglycerol assay is performed either simultaneously to allow a quicker result or as a second test if there is any contamination.5 The average sensitivity and specificity of the phosphatidylglycerol assay are 86% and 55%, respectively. The disadvantage of the phosphatidylglycerol assay is that it is a late indicator of fetal lung maturity, which limits our ability to deliver the mature fetus at its earliest possible gestation.5
One of the newer tests available to evaluate the fetal lung maturity status is the TDxFLM II assay (Abbott Laboratories, Abbott Park, IL). It is a rapid, non–labor-intensive, and reproducible test that is widely used in many hospitals across the country.2 The TDxFLM II assay requires 0.25 mL of amniotic fluid and takes approximately 30 minutes to complete.6 It measures the surfactant content in amniotic fluid while using the amniotic fluid albumin as an internal reference. Thus, a surfactant albumin ratio is determined. With the newer reagent (TDxFLM II), a ratio of 55 mg surfactant per gram of albumin or greater has been considered mature.6 Values between 40 and 55 are considered intermediate and would require additional tests, such as the phosphatidylglycerol assay. Values less than 40 are immature. Applying these values, approximately 20% of infants with an intermediate value and 42% with an immature value will develop respiratory distress syndrome.6 The differences between the newer reagent and the older reagent are in the packaging, the lower sample volume required, and the value of the low control being 25 mg/g instead of 15 mg/g.7 The only limitation of this test is its unreliability in the face of blood or meconium contamination.8,9
The purpose of this study is to evaluate the effect of meconium on the TDxFLM II assay, and, if the value is altered by meconium contamination, to aid in the interpretation of the test result.
|
MATERIALS AND METHODS |
|---|
|
TOP
ABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
After all samples were collected, they were thawed in a 37°C water bath for 10 minutes. To evaluate for the presence of meconium in each sample, 50 µL of fluid was taken from each tube, and the absorption at 405 nm was recorded on an EL312e Microplate reader (Bio-Tek Instruments Inc, Winooski, VT). This protocol is consistent with previous work by Svigos et al.10 Meconium was determined to be present if a value greater than 0.5 was seen at 405 nm. This would be equivalent to a spike seen on a scan because most values ranged between 0.12 and 0.2.
Each sample was divided into 4 separate tubes. The first tube had no additive, and a baseline TDxFLM value was obtained. The remaining 3 tubes were contaminated with fresh meconium. The meconium was obtained from the first bowel movement of a neonate who had a similar gestational age to that of the fetus when the specimen was collected. None of the neonates were noted to have a medical condition that would affect the consistency of the meconium, such as cystic fibrosis. Each amniotic fluid sample was weighed and the sample was contaminated with meconium at 1%, 5%, and 10% concentrations by weight. This is roughly equivalent to light, moderate, and thick meconium, respectively.11 The samples were then mixed thoroughly until homogeneous.
All samples were analyzed on an Abbott TDxFLM automated fluorescent polarization analyzer (Abbott Laboratories) with TDxFLM II reagent. A single operator performed the testing on all samples on the same day. The analyzer was calibrated, and 3 controls were run before testing the samples.
We considered a relative change in the TDxFLM II value caused by meconium contamination compared with the uncontaminated baseline value as our primary measure. Wong et al1 estimated the standard deviation of this relative change to be 13%, or 7.15, based on the minimum mature value of 55. Fantz et al12 concluded that a value as low as 45 could be used for maturity, yielding a sensitivity and specificity of 100% and 90%, respectively. Therefore, we felt comfortable selecting a change in value of 5 as significant. On the basis that 55 is the value at which the TDxFLM II reaches maturity and assuming a significant change in value of 5 and a standard deviation of 7.15, with a power of 80% and a 2-sided alpha of 0.05, the necessary sample size was determined to be 16 specimens.
|
RESULTS |
|---|
|
TOP
ABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
|
Table 2 represents the TDxFLM II results obtained. In all cases, the TDxFLM II value decreased from the baseline value once meconium was added. The mean values for the baseline, 1%, 5%, and 10% meconium concentrations are 51.29, 25.71, 14.89, and 4.99, respectively. There was no consistent rate of decrease that correlated with the percentage of meconium added. In many cases, the TDxFLM II value was decreased below the lower limit of detection by the analyzer. Fig. 1 is a graphic representation of the data presented in Table 2.
|
|
Because we followed the manufacturers established protocol for the collection and analysis of TDxFLM II values, a control for freezing was not warranted. However, as a point of interest, sample "U" was collected on the day of preparation and analysis and was used to verify that there were no differences in values as a result of freezing the sample. Therefore, half of sample "U" went through the same protocol as the other samples and the other half went through the protocol minus freezing. The values before and after freezing are 59.22 and 57.52, respectively.
|
DISCUSSION |
|---|
|
TOP
ABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Research has shown that contamination with small amounts of blood or meconium has negligible effects on the fetal lung maturity value.2 Grenache et al9 went a step further and found that blood contamination up to 0.03 x 1012 erythrocytes/liter is acceptable. In the event of gross contamination with blood, the value of the TDxFLM II is elevated.9 Therefore, an elevated value may be falsely interpreted as mature. However, Grenache found this to be significant only at values less than 40.9 Previously, a value of 75 was used for maturity in the face of grossly visible blood.1 This study by Wong et al1 hinted that meconium may lower the TDxFLM value, making a mature fetal lung test appear to be immature. However, only 3 specimens with meconium were used in the study. This finding suggests a possible scenario of delaying the delivery of a mature fetus secondary to an error in the laboratory test.
Of the 21samples collected, 17 were used to reach our conclusion. The remaining 4 samples were kept in the study as points of interest. The 3 samples found to be contaminated with meconium by spectrophotometry before processing had less than 1% of meconium contamination. Because it has already been shown that small amounts of meconium do not affect the TDxFLM value, an argument could be made for keeping these values in the study. These samples did behave similarly to those that were not contaminated. As previously mentioned, the fourth sample did not show a difference with freezing.
An interesting observation of this study was that a few of the samples contaminated with 5% meconium and the majority of those containing 10% meconium had values that were so low as to not be detected by the TDx analyzer. As one might predict, the lower the baseline value, the more likely this was to occur. However, this was not always observed. Rather, as the values approached the lower limits of detection by the TDx analyzer, the values began to show some variation. We observed that, at the lower limits of detection, some of the values were higher in the samples with 10% meconium than those with 5% meconium. In reality this is probably not the case, but rather, the analyzer is unable to accurately record a value at its lower levels of detection.
The results obtained from our study confirm the earlier predictions made by Wong et al.1 Meconium contamination lowers the TDxFLM II value. Wong also mentioned that plasma lowers the TDxFLM value. These observations make sense because both meconium and plasma contain albumin. Since the TDxFLM value is a measure of the surfactant albumin ratio in the amniotic fluid, one can see how any material containing albumin would lower this value. However, whole blood contains albumin and yet we see an elevation at lower values. This may be the result of the phospholipids in blood mimicking surfactant.
The manufacturer has stated that the TDxFLM value is invalid when the sample is contaminated with meconium. However, no data regarding this statement exist. As we have shown, meconium contamination does not necessarily invalidate this test. It can be said that, if a TDxFLM value is obtained in the face of meconium and returns as a mature value, then the fetal lungs are mature. An argument could be made for delivery in the face of meconium regardless of the result. However, if the meconium is light and there are no signs of maternal or fetal distress, it has been shown that meconium alone is a poor predictor of fetal or neonatal jeopardy.20 We feel that this time could best be used to get the benefits of steroids. Therefore, our findings are clinically useful for those samples containing light meconium. The only real disadvantage to using the TDxFLM II in the face of meconium is that there will be some values that are read as immature, when in fact they are mature. In this case, the addition of a phosphatidylglycerol assay might be helpful.5 In cases of moderate-to-thick meconium, most physicians are going to deliver regardless of the TDxFLM II value.
We had also entertained the idea of finding a value that could be used in the face of meconium contamination. We realized that 3 potential values would be possible. As stated above, a mature value taken from a meconium-contaminated environment would still be mature, and thus a value would not be needed. But, one could see the potential benefits for those TDxFLM II values that are near maturity. The only problem is that light, moderate, and thick meconium is subjective and, therefore, 3 values would be limiting. One solution would be to determine a corrected value. One potential method of determining the corrected value would be to find the optical density of the meconium-contaminated sample, and using the proper formula, a corrected value could be realized. The only down side is that now the TDxFLM II becomes a more time-consuming test for what is already going to be a small proportion of scenarios. And, as previously stated, in the face of moderate or thick meconium, it is likely that the patient will be delivered. In other words, to find the corrected value for the TDxFLM II in a meconium-contaminated sample is not clinically useful.
In conclusion, the TDxFLM II assay is one of the best available tests in determining the lung maturity of the fetus. It has the best sensitivity and good specificity when compared with the other tests available. It only takes 30 minutes to perform, is fairly inexpensive, requires little technical expertise, and has little intra- and interlaboratory variation. It has already been proven to be valid in the face of blood and, unlike the lecithin/sphingomyelin ratio, can be used in the face of meconium contamination. With our findings, it is clear that meconium lowers the TDxFLM II value. We, therefore feel safe in concluding that a mature TDxFLM II result is trustworthy in a meconium-contaminated specimen.
|
Footnotes |
|---|
Received February 26, 2004. Received in revised form June 26, 2004. Accepted July 9, 2004.
doi:10.1097/01.AOG.0000140693.06795.5d
|
REFERENCES |
|---|
|
TOP
ABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
2. Russell JC, Cooper CM, Ketchum CH, Torday JS, Richardson DK, Holt JA, et al. Multicenter evaluation of the TDx test for assessing fetal lung maturity. Clin Chem 1989;35:1005–10.
3. Tabsh K, Brinkman CR 3rd, Bashore R. Effect of meconium on amniotic fluid lecithin: sphingomyelin ratio. Obstet Gynecol 1981;58:605–8.
4. Longo SA, Towers CV, Strauss A, Asrat T, Freeman R. Meconium has no lecithin or sphingomyelin but affects the lecithin/sphingomyelin ratio. Am J Obstet Gynecol 1998;179:1640–2.[Medline]
5. Towers CV, Garite TJ. Evaluation of new amniostat-FLM test for the detection of phosphatidylglycerol in contaminated fluids. Am J Obstet Gynecol 1989;160:298–303.[Medline]
6. Kesselman EJ, Figueroa R, Garry D, Maulik D. The usefulness of the TDx/TDxFLx fetal lung maturity II assay in the initial evaluation of fetal lung maturity. Am J Obstet Gynecol 2003;188:1220–2.[Medline]
7. Edwards RK, Duff P, Ross KC. Amniotic fluid indices of fetal pulmonary maturity with preterm premature rupture of membranes. Obstet Gynecol 2000;96:102–5.
8. Steinfeld JD, Samuels P, Bulley MA, Cohen AW, Goodman DB, Senior MB. The utility of the TDx test in the assessment of fetal lung maturity. Obstet Gynecol 1992;79:460–4.[Medline]
9. Grenache DG, Parvin CA, Gronowski AM. Preanalytical factors that influence the Abbott TDx Fetal Lung Maturity II assay. Clin Chem 2003;49:935–9.
10. Svigos JM, Stewart-Rattray SF, Pridmore BR. Meconium-stained liquor at second trimester amniocentesis: is it significant? Aust N Z J Obstet Gynaecol 1981;21:5–6.[Medline]
11. Weitzner JS, Strassner HT, Rawlins RG, Mack SR, Anderson RA Jr. Objective assessment of meconium content of amniotic fluid. Obstet Gynecol 1990;76:1143–4.
12. Fantz CR, Powell C, Karon B, Parvin CA, Hankins K, Dayal M, et al. Assessment of the diagnostic accuracy of the TDx-FLM II to predict fetal lung maturity. Clin Chem 2002;48:761–5.
13. Ruch AT, Lenke RR, Ashwood ER. Assessment of fetal lung maturity by fluorescence polarization in high-risk pregnancies. J Reprod Med 1993;38:133–6.[Medline]
14. Barkai G, Reichman B, Modan M, Goldman B, Serr DM, Mashiach S. The influence of abnormal pregnancies on fluorescence polarization of amniotic fluid lipids. Obstet Gynecol 1988;72:39–43.
15. Livingston EG, Herbert WN, Hage ML, Chapman JF, Stubbs TM. Use of the TDxFLM assay in evaluating fetal lung maturity in an insulin-dependent diabetic population. Obstet Gynecol 1995;86:826–9.[Abstract]
16. Herbert WN, Chapman JF, Schnoor MM. Role of the TDxFLM assay in fetal lung maturity. Am J Obstet Gynecol 1993;168:808–12.[Medline]
17. Bonebrake RG, Towers CV, Rumney PJ, Reimbold P. Is fluorescence polarization reliable and cost efficient in a fetal lung maturity cascade? Am J Obstet Gynecol 1997;177:835–41.[Medline]
18. Hamilton PR, Hauschild D, Broekhuizen FF, Beck RM. Comparison of lecithin: sphingomyelin ratio, fluorescence polarization, and phosphatidylglycerol in the amniotic fluid in the prediction of respiratory distress syndrome. Obstet Gynecol 1984;63:52–6.
19. Hagen E, Link JC, Arias F. A comparison of the accuracy of the TDxFLM assay, lecithin-sphingomyelin ratio, and phosphatidylglycerol in the prediction of neonatal respiratory distress syndrome. Obstet Gynecol 1993;82:1004–8.
20. Miller FC, Sacks DA, Yeh SY, Paul RH, Schifrin BS, Martin CB Jr, et al. Significance of meconium during labor. Am J Obstet Gynecol 1975;122:573–80.[Medline]
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
