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Describe the potential causes for, and appropriate clinical chemistry investigations of an infant presenting with failure to thrive. Failure to thrive (FTT) is defined as a current weight or rate of weight gain significantly below that expected for children of similar age and sex. It usually refers to growth below the 3rd or 5th percentiles or a change in growth rate that has crossed two major percentiles in a short time. Potential causes of FTT in infants FTT occurs due to 3 mechanisms or a combination of them. It could be due to failure to provide enough calories, inability of the body to absorb enough calories or due to increased energy expenditure. Causes of FTT are divided into organic and non organic causes. Non organic FTT occurs when no underlying medical condition is found and it is usually due to psychosocial and/or nutritional deprivation. This could be due to maternal depression ,child neglect, improper formula preparation, delayed introduction of age appropriate foods etc. This will lead to a child not getting enough calories to thrive. Organic FTT occurs when an underlying medical condition causing poor growth is diagnosed. Common organic causes of FTT in infancy are listed in the table below Common organic causes of FTT in infancy MALABSORPTION Intestinal causes (Food intolerance eg.lactose, Celiac disease) Pancreatic insufficiency(cystic fibrosis) INHERITED METABOLIC DISEASES CHRONIC DISEASE Pulmonary Cardiac Renal Liver ENDOCRINE DISEASE DM,hypothyroidism,GH deficiency,hyperthyroidism CHRONIC/RECURRENT INFECTIONS Eg.immunodeficiency,HIV,recurrent UTI OTHERS: Gastroesophageal reflux Pyloric stenosis Neurologic disease Malignancy Insufficient calories intake may occur due to the above mentioned non organic causes or could be due to difficulties with feeding as in motor neuron diseases or due to regurgitation and vomiting which may occur with gastroesophageal reflux disease, pyloric stenosis or inborn errors of metabolism. Common causes of malabsorption in infancy are food intolerance, food allergy, celiac disease and cystic fibrosis. Chronic diseases as well as recurrent infections and inborn errors of metabolism may cause failure to thrive due to improper energy utilization and energy over expenditure. Endocrine diseases such as DM, hypothyroidism and hypopituitarism may cause FTT but they are more common later during childhood. Clinical chemistry investigations: Non organic causes of FTT are more common than organic causes especially in developing countries. Nevertheless, it is considered appropriate to screen infants with FTT for common organic causes which have to be ruled out before a psychosocial/nutritional FTT is diagnosed. In addition, proper psychosocial and nutritional history should be saught.The history and/or physical examination eg.cardiac murmur may point to a specific cause. Basic clinical biochemistry investigations plays a major role in identifying patients with chronic kidney or liver disease/RTA, screening and diagnosis of inborn errors of metabolism, Identifying few causes of malabsorption mainly cystic fibrosis and celiac disease. Basic biochemistry investigations may initially include RFT, LFT to rule out chronic liver or kidney disease. Raised liver enzymes may be due to many causes including inborn errors of metabolism as well as chronic infections and malignancies. Isolated rise of ALP without evidence of liver disease may point to vitamin D deficiency. Low albumin may point to malnutrition/malabsorption but it could be low in chronic disease as well as in kidney disease with proteinuria. Moreover, it has a half life of 21 days and needs weeks to go down in cases of malnutrition or liver disease.Doing serum prealbumin is found to be a better indicator of malnutrition since it has a half life of 2 days and if it fails to increase after a trial of feeding then further investigations to rule out other causes is appropriate. CRP should be done along with prealbumin to rule out inflammation as a cause of low prealbuminas. Doing other markers of malnutrition at this stage is appropriate. These include serum calcium, magnesium, ferritin, folic acid and vitamin B12. In addition, performing Urinalysis for protein, bilirubin, reducing substances and PH helps screen for nephropathy and some inborn errors of metabolism. Additional biochemistry tests may be done as indicated from the history and examination. 1.patients presenting with chronic diarrhea (or steatorrhoea), flatulence or abdominal pain, a stool sample may be taken for reducing substances(for lactose intolerance) and pH measurement.It may be analyzed for elastase if pancreatic insufficiency due to cystic fibrosis is suspected. If cystic fibrosis is suspected then a chloride sweat test is performed. Celiac disease is not uncommon to present in this age group so endomysial antibodies or transglutaminase antibodies should be done to screen for celiac disease.Those tests have been shown to be very sensitive in diagnosing celiac disease. Serum IgA should be done in patients with chronic diarrhea/ recurrent GIT infections. 2.Screening for inborn errors of metabolism is done for Patients with abnormal LFTS, non specific symptoms, hypoglycemia or suspected metabolic disorder. Serum ammonia and lactate along with blood gas analysis are first line investigations. Further metabolic workup including plasma and urine amino acids/organic acids is done accordingly. 3. Immunoglobulins are done in patients with recurrent infections. They could be low in malnutrition. 4. Endocrine causes are uncommon to present with FTT at this age group especially that neonatal screening for hypothyroidism has been established. Nevertheless, TFT should be done if thyroid disease is suspected . Finally, in addition to clinical biochemistry investigations it is worth mentioning other complementary tests from other labs eg. FBC, urine and stool cultures for diagnosing gut infestations,genetic testing for CF, gastroesophageoscopy and sigmoidoscopy with biopsy to diagnose intestinal causes of malabsorption ect. Conclusion: FTT in an infant is commonly due to non organic causes. Thorough nutritional and psychosocial history along with physical examination is important to guide management but basic clinical chemistry investigations may help rule out and follow up treatment of common organic causes (mainly metabolic diseases, cystic fibrosis, celiac disease, biochemical consequences of malnutrition) and guide further management. It may help screen for malabsorption but further evaluation by endoscopic means and biopsy facilities is more appropriate.