and alkaline phosphatase show reduced activity levels in the AF of various pathologic pregnancies, such as trisomy 18192 and 21.193 Elevated activity of alkaline phosphatase has been observed in intrauterine fetal death, abdominal wall defect, Meckel syndrome, hydrops fetalis, and genital anomaly.186 Elevated alkaline phosphatase activity in third‐trimester AF is often associated with fetal disorders.
Autosomal recessive villus atrophy syndrome is characterized by an atrophy of intestinal villi and reduced disaccharidase activities in the intestinal mucosa.196 The disaccharidase activities would be expected to be low or deficient in the AF of an affected fetus. Normal disaccharidase activities in the AF were found in one case, and the newborn was subsequently noted to be phenotypically normal.172
Trehalase activity in AF has been used to detect renal anomalies. Morin et al.166 reported elevated trehalase/palatinase (or lactase) activity ratios in the AF of a fetus with polycystic kidney disease type II and two fetuses with congenital nephrotic syndrome of the Finnish type. These ratios were used as indices of the presence of renal trehalase in AF because palatinase and lactase are exclusively of intestinal origin.166 Fetuses with renal congenital disease and degeneration of kidney tissue can be expected to release higher than normal levels of renal trehalase activity in the amniotic cavity.
Miscellaneous biochemical constituents and other characteristics of amniotic fluid
Various nonenzymatic constituents and characteristics of AF are listed in Table 3.2. Elevated values of biochemical constituents in AF may be nonspecific, such as 5‐hydroxyindole‐3‐acetic acid in a fetus with Cornelia de Lange syndrome353 or acid‐soluble glycoproteins in AF in CF.261 Reduced amounts of 5‐hydroxyindole‐3‐acetic acid,271 and increased amino acids (especially methionine, isoleucine, leucine, tyrosine, and phenylalanine) have been observed in the AF of fetuses with NTDs.354 Two likely reasons are transudation across the defect and a nonspecific reflection of fetal distress with hypoxia. Certain other constituents of cell‐free AF, such as 17‐ketosteroids, are especially relevant to prenatal diagnosis.
Table 3.2 Biochemical constituents and other characteristics of amniotic fluid (see text for additional sources)
| Biochemical constituent/characteristic | Selected reference(s) |
|---|---|
| Acetylcholine receptor | 197 |
| Acetylcholinesterase (AChE) | 198 |
| Acid–base | 199, 200 |
| ADAM‐8 | 5, 201 |
| Adrenomedullin | 202 |
| α1‐Antitrypsin | 205 |
| α‐Fetoprotein | this chapter |
| α1‐Macroglobulin | 206 |
| Albumin | 203 |
| Alkaline phosphatase | 189, 204 |
| Amino acids | 149, 207 |
| Amylase | 208 |
| Androgens | 209 |
| Angiogenin | 210 |
| Anticardiolipin antibodies | 211 |
| Antithrombin | 206 |
| Antiviral effect | 212 |
| Apolipoprotein | 213 |
| Apolipoprotein A | 214 |
| Arginosuccinic acid | 215 |
| Arylsulfatase A | 216 |
| Atrial natriuretic factor | 217 |
| Bacteria | 218 |
| Bacterial growth, inhibitory effect | 219, 220 |
| Basement membrane protein | 67 |
| β‐Endorphin | 221 |
| β‐Hydroxybutyrate | 222 |
| β2‐Microglobulin | 223 |
| Bile pigments | 183 |
| Bilirubin | 224 |
| Blood group substances | 225, 226 |
| Blood urea nitrogen | 227 |
| Cadmium | 228 |
| Candida albicans | 229 |
| Calcium | 230 |
| Calgranulins | 231 |
| Carcinoembryonic antigen | 232 |
| Carnitine | 233 |
| Catecholamines | 234 |
| Chemokines | 235 |
| Ceruloplasmin | 203 |
| Cholesterol | 89 |
| Chorionic gonadotropin | 209 |
| Chorionic somatomammotropin | 209 |
| Citric acid | 236 |
| Citrulline | 215 |
| Clara cell protein | 153 |
| Complement | 203 |
| Copper | 238 |
| Cortisol | 239 |