HKCOG Guidelines Guidelines of Antenatal Thalassaemia Screening published by The Hong Kong College of Obstetricians and Gynaecologists A Foundation College of Hong Kong Academy of Medicine Number 8 October 2003 1 INTRODUCTION Thalassaemia (-thal) is the commonest single-gene defect in the world. Homozygous ƒั0-thal is associated with anaemia and pregnancy complications, including hypertension, antepartum haemorrhage, postpartum haemorrhage, heart failure, renal failure and hydrops fetalis1. Children with ƒา-thal major need lifelong transfusion and iron chelation. Couples who are both ƒั0-thal or ƒา-thal minor run 25% risk of having homozygous offsprings. 2 MOLECULAR BASIS OF THALASSAEMIA Two ƒั genes have been identified on the ƒ๊ƒ{ƒั globin gene cluster on chromosome 16. In Hong Kong, deletion of ƒั genes (deletion ƒั-thal) and non-deletional ƒั-thal exist in a ratio of approximately 3 to 12. The majority of deletion ƒั-thal involves deletion of both ƒั genes on the same chromosome (in cis) (ƒั0-thalassaemia). Occasionally there is loss of the entire ƒ๊ƒ{ƒั gene cluster. Loss of one ƒั gene in the gene cluster is designated ƒั+-thalassaemia, and offsprings of these individuals will not suffer from serious anaemia. The commonest non-deletional ƒั-globin gene mutation in Hong Kong is Hb Constant Spring (Hb CS). This is caused by a termination codon mutation resulting in the production of unstable ƒั globin chain. In general, the degree of anaemia depends on the number of functional ƒั genes left. ƒา-thal is due to mutation in or around the ƒา gene. Over 90 ƒา gene mutations have been discovered worldwide. A local study revealed that 87% of ƒา-thal was related to four common forms of mutations3. The mutations are classified depending on the degree of reduced ƒา globin chain synthesis: ƒา0 results in total loss of ƒา chain synthesis, while synthesis is only reduced but not absent in ƒา+. Thus homozygous ƒา0-thal (ƒา0/ƒา0) results in life-long transfusion dependency. (A glossary of terms is included at the end of the Guideline for easy reference.) 3 PREVALENCE Thalassaemia is prevalent in the Mediterranean, through Africa and the Middle East to the Far East. Using different criteria for screening, the studies on the prevalence in Hong Kong revealed that ƒั-thalassaemia (ƒั-thal) and ƒา-thalassaemia (ƒา-thal) exist in 3 - 5% and 2.5 กV 3.1% of the local population respectively 4-9. With local delivery of 50,000 a year, and an estimate of 4.3% carrier of ƒั-thal and 2.8% carrier of ƒา-thal 9, the number of infants with homozygous ƒั0-thalassaemia is estimated to be 23; and the number of infants with ƒา-thal major is 10. Because of the risks of adverse pregnancy outcome among the affected pregnancies, screening for at-risk pregnancies is required. In Hong Kong, thalassaemia screening has been offered to all pregnant women at antenatal booking at every public hospital and every Maternal and Child Health Centre (MCHC), from July 2000 onwards. Currently, it is estimated that over 95% of all deliveries in public hospitals have undergone thalassaemia screening. Corresponding information from the private sector is lacking. This guideline applies to screening for the majority of the local community, which consists of southern Chinese. For other ethnic populations, the guideline may not apply and full hematological study is needed to detect the at-risk hemoglobin disorders. 4 HAEMATOLOGICAL FINDINGS AMONG THALASSAEMIA CARRIERS (Table 1) An understanding of the haematological changes in thalassaemia carriers provides basis to develop simple screening criteria. Haemoglobin (Hb) level is usually normal among non pregnant ƒั-thal and ƒา-thal carriers. In ƒั0-thalassaemia carriers, the MCV is below 80fl. Hb A2 is normal. Occasional Hb H granules may exist, but these may be absent. Hence ƒั0-thal carrier state cannot be excluded in patients with low MCV, in spite of normal haemoglobin pattern. In single ƒั-globin gene deletion (ƒัƒyƒ{thal) and ƒั-globin gene mutation (non deletion ƒัƒyƒ{thal) such as Hb Constant Spring, MCV ranges between 80 and 85fl. This finding may be significant if the partner carries ƒั0-thal, because Hb H disease or uncommon forms of thalassaemia genotypes may exist in the offsprings. The current antenatal thalassaemia screening programme is not tailored towards prediction of HbH disease. For ƒาƒnthal carriers, MCV is consistently below 80 fl. Hb A2 is raised10. The presence of low MCV with raised Hb A2 is diagnostic of ƒาƒnthal. However, Hb A2 level may remain normal among ƒาƒ{thal carriers with iron deficiency. Hence ƒาƒ{thal trait cannot be excluded in women with low MCV and normal Hb A2 levels, unless iron deficiency is excluded. Individuals with compound ƒั and ƒา thal trait also have MCV below 80fl. HbH inclusion bodies may be absent. Hb H granules should therefore not be used as a means to detect concomitant ƒั0-thal in ƒา-thal carriers11. DNA analysis to exclude ƒั0-thal would be necessary for such individuals. Table 1: Haematological parameters of thalassaemia carriers Hb (g/dL) MCV (fL) A2 (%) HbH bodies * ƒั0-thal (Pregnant females) n = 408 10.82 (8.3 กV 13.7) 68.8 (61.1 กV 78.9) 2.39 (2 กV 3.24) occasional or absent * ƒั0-thal (Males) n = 423 13.84 (10 กV 17) 68.45 (58.8 กV 79.5) 2.37 (2 กV 3) occasional or absent ** ƒั+-thal (Males and non pregnant females) n = 70 13.5 (11.7-15.8) 81.5 (72.6-92.5) 2.7 (2.4 กV 3) Absent * ƒาƒnthal minor (Pregnant females) n = 478 10.21 (6.9 กV 13.2) 66.1 (54-78) 5.48 (3.6-6.7) Absent * ƒาƒnthal minor (Males) n = 570 13.17 (8.0-16.0) 65.2 (52.6-77.5) 5.54 (4.0-6.9) Negative * ƒั-ƒาƒnthal (Males and females) n = 26 13.00 (9.3-16.6) 71.81 (64.7-78.9) 5.55 (4.8-6.5) occasional (absent in 11 cases) Notes: 1. Table is compiled from data provided by Professor V Chan from the Division of Molecular Medicine, Department of Medicine, the University of Hong Kong (*), and by Dr. E Ma from the Division of Hematology, Department of Pathology, the University of Hong Kong (**) 2. Hb, MCV, A2 are tabulated as mean values, with range indicated in brackets. 5 PARAMETER FOR THALASSAEMIA SCREENING IN PREGNANCY Given the heterogeneity of thalassaemia, a simple screening test for every variety of thalassaemia has not been discovered. A practical approach is to screen for the common and clinically important thalasssaemias in the locality. The goal is to identify the couples at risk and to detect in early pregnancy the severe forms of the disorder in the fetus, ie, homozygous ƒั0-thalassaemia and ƒา-thal major, in order to avoid morbidity and mortality. Both MCV and MCH have been identified as appropriate measures to screen for ƒั- and ƒา-thal. MCV is more often used for the purpose, while MCH has the theoretical advantages over MCV because it is more stable over time. Both parameters are highly accurate. Although red cells stored in room temperature tend to swell over time so that the sensitivity of MCV may decrease when stored blood is screened, this may not be a concern in Hong Kong as our samples need not travel long distances. In Hong Kong, MCV has been adopted for screening thalassaemias. Different cutoffs in MCV have been used in various screening programmes 5,9. Most have adopted a cutoff between 76 fL and 82 fL. With MCH, 27pg seems to be the most acceptable cutoff. By genotyping archived samples from school children up to MCV of 85 fL, Ma et al 12 found that an MCV cutoff of 80 fL or MCH cutoff of 27 pg could detect all ƒั0-thal carriers and ƒา-thal carriers. The authors recommended taking these cutoffs as criteria for thalassaemia screening in Hong Kong. Since both MCV and MCH tend to increase in pregnancy, the cutoffs may not apply to antenatal screening without loss in sensitivity. Currently the joint antenatal thalassaemia screening programme between public hospitals and the MCHC adopts an MCV cutoff of 80 fL, while some local haematology laboratories adopt a cutoff of 82 fL. 6 TIMING OF SCREENING Early experience of antenatal thalassaemia screening involved mainly pregnancies before 18 weeks. Early screening allows adequate time for further workup and counselling of the couples. In case termination of pregnancy needs to be considered, timely diagnosis of thalassaemia major in the foetus is desirable. Since Hb Bartกฆs hydrops foetalis carries risks to the mother, antenatal thalassaemia screening should be offered to all pregnant women with unknown thalassaemia status, regardless of gestational age. 7 EDUCATION AND COUNSELLING AS AN INTEGRAL PART OF SCREENING Current practice of screening necessitates informed consent from patients. Education and information for patients is required before informed consent can be obtained. These may be provided to patients through various means, including information pamphlets, group counseling, video display, posting on website or explanation in person. The information appropriate for the purpose is listed in appendix 1. Detailed pre-test counselling to patients is usually not necessary unless the patient is particularly anxious. In cases of screen-positives or known carriers, further counselling is appropriate to allay anxiety and to prepare patients and their families psychologically. While admitting the chance that the fetuses may be affected adversely, the couples should understand that the pregnancies are more likely to be normal and termination of pregnancy before further diagnostic procedure is not necessary. 8 FURTHER WORKUP FOR SCREEN-POSITIVES AND LOW MCV COUPLES (Appendix 2) Screen-positives are women whose MCV and/or MCH fall below the cutoff value. Among ethnic Southern Chinese, the majority has one of the following conditions: ƒั0-thal, ƒา-thal trait or iron deficiency. Tests for Hb H inclusion bodies and elevation in Hb A2, and iron study, should be performed. Presence of Hb H inclusion bodies and elevation in Hb A2 (>3.5%) are diagnostic of ƒั0-thal and ƒา-thal respectively. Co-existent ƒา-thal cannot be totally excluded if there is iron deficiency, and ƒั0-thal cannot be excluded in any individual with low MCV or low MCH, absent Hb H inclusion bodies, and normal ferritin level. Workup on the partners begins with MCV and/or MCH estimation for screening of ƒั0-thal and ƒา-thal trait. Save for some special situations to be discussed later, levels above the respective cutoff virtually excludes serious thalassaemia affecting the foetus. If the partners are screen-negative, no further workup is necessary. Partners who are screened positive should be tested for presence of Hb H inclusion bodies, elevation of Hb A2, and iron study. The purpose is to identify whether the partners have thalassaemia affecting the same globin gene as the screen positive women, ie, whether they are ƒั-ƒั couples or ƒา-ƒา couples. Since 7% of ƒา thal carriers were found to be compound ƒั- and ƒา-thal heterozygotes13, couples discordant for ƒั- and ƒา thalassaemia (ƒั-ƒา couples) should be counselled and offered DNA study to exclude co-existence of ƒั thal in the ƒา thal partner. According to a review of the screening programme between public hospitals and MCHC, testing was required in 0.89% (133 out of the 15013 couples) of the pregnancies screened at MCHC between July 2000 and June 2001. Due to the high prevalence of thalassaemias in Hong Kong, clinically important thalassaemias due to rare thalassaemia genotypes has been reported14-16. If resources allow, partners of known thalassaemia carriers, regardless of MCV, may be referred to haematologists for investigation of rare thalassaemias and identification of pregnancies at risk of Hb H disease. For ƒั-ƒั couples, ƒา-ƒา couples and ƒั-ƒัƒา couples, detailed counselling and further prenatal testing should be offered. Since ƒั0-thal cannot be excluded in individuals with low MCV despite normal Hb and iron studies, low MCV couples (with normal iron studies and normal Hb pattern) should be managed as potential ƒั-ƒั couples. Explanation on the significance of ƒั-ƒั couples, ƒา-ƒา couples and ƒั-ƒัƒา couples may be carried out by most practicing obstetricians (appendix 3). Experienced personnel and laboratories should perform further prenatal testing for these couples. Currently the workup involves special ultrasound examination, or sampling of amniotic fluid cells, chorionic villi or foetal blood. The risk of miscarriage (fetal loss) is around 0.5% following amniocentesis, and around 1% following chorionic villus sampling or fetal blood sampling. Trained personnel can safely and reliably perform these procedures. The samples should then be sent to established laboratories experienced in handling fetal samples and running the special DNA studies. 9 SUMMARY 9.1 Thalassaemia is highly prevalent in Hong Kong. Antenatal thalassaemia screening provides an opportunity for early detection of Hb Bartกฆs disease and ƒา-thal major. 9.2 Antenatal thalassaemia screening aims at identifying ƒั-ƒั couples, ƒา-ƒา couples or ƒั-ƒัƒา couples whose off-springs are at risk of having Hb Bartกฆs disease and ƒา-thal major. It should be offered to all pregnant women, regardless of gestational age, in particular, for prenatal detection of Hb Bartกฆs disease. 9.3 Information on the significance of thalassaemia should be provided to pregnant women as an integral part of the antenatal thalassaemia screening programme. 9.4 Measurement of MCV and/or MCH is appropriate for antenatal thalassaemia screening. An MCV cutoff of < 80 fL, or an MCH cutoff of <27 pg is recommended for antenatal thalassaemia screening. 9.5 Thalassaemia screening by measuring MCV and /or MCH should be offered to partners of pregnant women who are screen-positive. 9.6 In areas with high prevalence of thalassaemia (such as Hong Kong), haemoglobin study to identify presence of occasional H granule and elevation in Hb A2, and iron study should be arranged for pregnant women who are screen-positive. Haemoglobin study and iron study for the partners should also be arranged for women known to be ƒั0-thal or ƒา-thal carriers. 9.7 Couples identified as ƒั-ƒั couples, ƒา-ƒา couples and ƒั-ƒัƒา couples should be counselled and offered prenatal diagnostic tests to exclude Hb Bartกฆs disease and ƒา-thal major affecting the foetus. 9.8 Prenatal work-up for low MCV couples, ƒั-ƒั couples, ƒา-ƒา couples and ƒั-ƒัƒา couples should be carried out by personnel and laboratories with experience in the procedures. REFERENCE LIST 1. Liang ST, Wong VCW, So WWK, Ma HK, Chan V, Todd D. Homozygous alpha-thalassaemia: Clinical presentation, diagnosis and management. A review of 46 cases. Br J Obstet Gynaecol 1985; 92:680-684. 2. Chen FE, Ooi C, Ha SY, Cheung BM, Todd D, Liang R, Chan TK, Chan V. Genetic and clinical features of hemoglobin H disease in Chinese patients. N Engl J Med 2000; 343:544-550. 3. Chan V, Chan TK, Chebab FF, Todd T. Distribution of ƒา-thalassemia mutations in South China and their association with haplotypes. Am J Hum Genet 1987;41:678-685. 4. Todd D, Lai MCS, Braga CA, Soo HN. Alpha-thalassaemia in Chinese: cord blood studies. Br J Haemat 1969; 16:551-556. 5. Ghosh A, Woo JSK, Wan CW, MacHenry C, Wong V, Ma HK, Chan V, Chan TK. Evaluation of a prenatal screening procedure for ƒา-thalassaemia carriers in a Chinese population based on the mean corpuscular volume (MCV). Prenat Diagn 1985;5:59-65. 6. Chan V, Chan TK, Cheng MY, Kan YW, Todd D. Organization of the ƒ๊ƒ{ƒั genes in Chinese. Br J Haematol 1986 Sept; 64(1):97-105. 7. Lam TK, So LY, Poon SH, Leung KW, Feng CS. Prevalence of ƒา-thalassaemia trait in Hong Kong children. The Hong Kong Journal of Paediatrics 1989; 6:119-121. 8. Lau YL, Chan LC, Chan YYA, Ha SY, Yeung CY, Waye JS, Chui DHK. Prevalence and genotypes of ƒั- and ƒา-thalassemia carriers in Hong Kong กV implications for population screening. N Engl J Med 1997;336:1298-1301. 9. Sin SY, Ghosh A, Tang LCH, Chan V. Ten yearsกฆ experience of antenatal mean corpuscular volume screening and prenatal diagnosis for thalassaemias in Hong Kong. J Obstet Gynaecol Res 2000;26:203-208. 10. Li AMC. Haematological findings in Hong Kong Chinese with ƒา-thalassaaemia trait. Journal of the Hong Kong Medical Association 1990;42:134-136. 11. Ma ESK, Chan AYY, Ha SY, Chan GCF, Lau YL, Chan LC. Screening for (--SEA) ƒั-globin gene deletion in ƒา-thalassaemia carriers and prevention of hydrops fetalis. Haematologica 2000;85:991-993. 12. Ma ESK, Chan AYY, Ha SY, Lau YL, Chan LC. Thalassaemia screening based on red cell indices in the Chinese. Haematologica 2001; 86:1286-1287. 13. Lam YH, Ghosh A, Tang MH, Chan V. The risk of ƒั-thalassaemia offspring of ƒา-thalassaemia carriers in Hong Kong. Prenat Diagn 1997;17:733-736. 14. Chan V, Chan TK, Liang ST, Ghosh A, Kan YW, Todd D. Hydrops fetalis due to an unusual form of Hb H disease. Blood. 1985 Jul;66(1):224-8. 15. Chan V, Chan VWY, Tang M, Lau K, Todd D, ChanTK. Molecular defects in Hb H hydrops fetalis. Br J Haemat 1997;96:224-228. 16. Chan LC, Ma SK, Chan AYY, Ha SY, Waye JS, Lau YL, Chui DHK. Should we screen for globin gene mutations in blood samples with mean corpuscular volume (MCV) greater than 80fL in areas with a high prevalence of thalassaemia? J Clin Pathol 2001;54:317-320 ACKNOWLEDGEMENT: This document was prepared by Dr Mary Tang, Dr KT Tse, Dr A Ghosh, Dr ST Liang, Dr SK Ma (Division of Haematology, Department of Pathology, HKU), Dr Amy Lai (Department of Health, HKSAR) and Professor V Chan (Division of Molecular Medicine, Department of Medicine, HKU) and was endorsed by the Council of the Hong Kong College of Obstetricians and Gynaecologists. This guideline was produced by the Hong Kong College of Obstetricians and Gynaecologists as an educational aid and reference for obstetricians and gynaecologists practicing in Hong Kong. The guideline does not define a standard of care, nor is it intended to dictate an exclusive course of management. It presents recognized clinical methods and techniques for consideration by practitioners for incorporation into their practice. It is acknowledged that clinical management may vary and must always be responsive to the need of individual patients, resources, and limitations unique to the institution or type of practice. Particular attention is drawn to areas of clinical uncertainty where further research may be indicated. APPENDIX 1 Appendix I Information for Pretest Education 1. Thalasaemia is one of the commonest hereditary blood disorders. 2. Incidence in Hong Kong: about 5% of local population carries the gene responsible for alpha thalassaemia (alpha thal); about 3% for beta thalassaemia (beta thal). 3. Most of these carriers are healthy (thalassaemia minor) and they would not know that they are carriers until appropriate blood tests have been performed for them. 4. The carrier status is inherited. It will not change with age. 5. If two carriers marry, their children have 25% chance of having serious anaemia (thalassaemia major). Children with ƒา-thal major will gradually develop severe anaemia a few months after birth. Infants with ƒั-thal major die in late gestation or shortly after birth. The pregnant women carrying ƒั-thal major fetuses may develop serious illness such as high blood pressure and convulsion in the latter part of pregnancy or after delivery. 6. Even if the partners are not carriers, pregnant carriers of thalassaemia require vitamin supplement to maintain generation of blood in her body. 7. The great majority of thalassaemia carriers in pregnancy can be detected by simple blood tests (screening tests for thalassaemia), which can be performed together with other antenatal blood tests. Similar blood tests may be arranged for the partners of screen-positives, to ascertain whether the partners are also thalassaemia carriers. 8. In some of the couples who are screen-positive, further blood testing (diagnostic tests) may be required to confirm or exclude whether they are thalassaemia carriers. 9. If both the husband and wife are thalassaemia carriers, prenatal diagnostic tests may be arranged for the pregnancy to exclude thalassaemia major in the fetus. 10. Prenatal diagnostic tests would be performed by personnel and laboratory with experience in the procedure. Appendix 3 Information for low MCV couples, alpha-alpha couples, beta-beta couples and alpha-alpha beta couples: 1. Thalasaemia is one of the commonest hereditary blood disorders. 2. Except for vitamin supplement (with folic acid) during pregnancy, no drug treatment is required for thalassaemia minor. 3. Thalassaemia minor does not weaken the body physically or mentally. It will not progress to thalassaemia major. 4. (For low MCV couples) Blood tests so far cannot confidently exclude that both you and your partner are alpha thalassaemia minor carriers. Further tests for both of you are required. 5. (For alpha-beta couples) Blood tests so far have shown that you and your partner are carriers of different types of thalassaemias. Because of the limitation of the simple tests, further tests are required to confidently exclude alpha thalassaemia carrier state for you or your partner. 6. If your partner also has thalassaemia minor of the same variety (alpha thal or beta thal), 25% of the offspring will have thalassaemia major. 7. (For alpha-alpha couples, alpha-alpha beta couples and low MCV couples) Infants with alpha thalassaemia major may die before or shortly after birth. The pregnant women carrying an affected fetus may develop serious illness such as high blood pressure and convulsion. 8. (For beta/beta couples) Babies with beta thalassaemia major will gradually develop severe anaemia a few months after birth. They will need life-long blood transfusion and special treatment (iron chelation) to minimize accumulation of iron in the organs due to repeated transfusion, otherwise they will die from consequences of iron overload. 9. For fetuses at risk of thalassaemia major, prenatal diagnostic tests may be arranged to exclude thalassaemia major affecting the fetus. These tests would be performed by personnel and laboratory with experience in the procedure. The tests are 100% definitive in most cases. Glossary of terms based on Weatherall & Clegg, The Thalassaemia Syndormes, 4th Edition, 2001 Thalassaemia: A group of genetic disorders (hereditary anaemia) characterized by a reduced rate of production affecting one or more globin subunits of the haemoglobin molecule. They are divided broadly into ƒัƒzƒnƒาƒzƒnƒืƒzƒnƒิƒาƒzƒnƒิ and ƒีƒืƒิƒา varieties, depending on which globin chain(s) is under-produced. ƒัƒ~-thalassaemia (ƒั-thal-1): Refers to the condition in which both ƒั-globin genes on the same chromosome (in cis) are deleted. The heterozygous genotype can also be designated as - -/ƒัƒั. ƒัƒy-thalassaemia (ƒั-thal-2): Refers to the condition when one of the two ƒั-globin genes is inactivated. The genotype is designated -ƒัƒ}ƒัƒั when one of the ƒั-globin genes is deleted, or ƒัƒฤƒัƒ}ƒัƒั when one of the ƒัƒ{globin genes is inactivated by a mutation (sometimes termed non-deletional ƒัƒyƒnƒ{thal). ƒาƒ~-thalassaemia: Refers to the condition in which no ƒา-globin chain is produced. ƒาƒyƒ{thalassaemia: Refers to the condition in which some residual ƒา-globin chain is produced (but less than normal). ƒาƒyƒyƒ{thalassaemia: Refers to the condition in which the defect in ƒา-globin chain production is particularly mild. Thalassaemia carrier / trait: Refers to individuals who are heterozygous for a thalassaemia mutation. These individuals are usually clinically asymptomatic but may give rise to offsprings affected by severe thalassaemia if the partner is also a carrier of the same type. HbH disease: Refers to individuals who are compound heterozygous for ƒัƒ~-thalassaemia and ƒัƒyƒ{thalassaemia, the latter of which can be deletional or non-deletional. Typical genotypes are therefore either three ƒั-globin gene deletion (--/-ƒั), or ƒั0-thalassaemia in association with non-deletional ƒั-globin gene mutation (--/ƒัƒฤƒั), for example HbCS (--/ƒัcsƒั). The clinical phenotype in these individuals is mild, and they are usually not transfusion dependent, although recent evidence shows that non-deletional HbH disease is associated lower haemoglobin, more splenomegaly and hence splenectomy, and more iron overload with age. Hb Bartกฆs hydrops fetalis: Refers to homozygous ƒั0-thalassaemia (i.e. four ƒั-globin gene deletion) and is usually associated with intrauterine death or stillbirth. ƒา-thalassaemia major (Cooleyกฆs anaemia): Refers to individuals who are homozygous for ƒา0-thalassaemia or compound heterozygous for ƒาƒ~ and ƒาƒyƒ{thalassaemia. These individuals usually present below the age of 1 year with severe anaemia that necessitates regular transfusion. They suffer from numerous disease complications due to iron overload, and have a limited life expectancy. ƒาƒ{thalassaemia intermedia: Refers to individuals who, although usually carrying two ƒา-thalassaemia alleles, either ƒาƒy/ƒาƒyƒnƒnorƒnƒาƒ~/ƒาƒyƒnƒzƒnare clinically less severely affected than ƒาƒ~-thalassaemia major. It could also be due to inheritance of other genetic factors that ameliorate the disease phenotype. HKCOG GUIDELINES NUMBER 8 (October 2003) 11