Specialised biochemical investigations Flashcards
assessing nutrition - macronutrients
protein: serum albumin although also affected by liver and renal function, hydration of patient, and falls rapidly as part of metabolic response to injury
blood glucose conc: maintained even in starvation, may see ketosis; hyperglycaemia is part of metabolic response to injury
lipids: fasting plasma triglyceride levels can indicate metabolism but affected by various processes, essential fatty acids measure for specific deficiencies; faecal fat for malabsorption but not always available
overall assessing macronutrients is not great, unlike for micronutrients
height and weight are most useful assessment of overall nutritional status at all life stages and often poorly recorded in patient notes
assessing nutrition - micronutrients
vitamins: some assays for direct blood levels but often functional assays exploiting that vits are often enzyme cofactors
major minerals measured in blood
trace elements often found in complex with proteins
assessing nutritional status needed pre and post op ideally, preop moreso only if >10% body weight loss with some weakness or other functional effect and any sepsis or whatever treated
vits needed in this order from most to least per day: vit C, niacin, vit E, B6, B2, B1, A, folate, K, D, B12
trace elements needed in this order from most to least per day: iron, zinc, manganese, copper, fluoride, molybdenum, iodine, selenium, chromium, vanadium
nutritional support
energy: harris-benedict equation takes BMR and applies activity factor to it and for ill patients adjust for weight loss and ongoing catabolic stresses; provide via mix of lipids and carbohydrates
nitrogen: RNI for protein is 0.75g/kg body weight/day. and note 1g nitrogen = 6.25g protein, 24hr urine sample can give more precise figure for required protein per day but cant use in renal or metabolically stressed patients
RDAs used for micronutrients
oral feeding wherever possible; next best option is tube feeding (NG, nasoduodenal, or gastrostomy tubes) but tube feeding may have mechanical obstruction and oseophageal erosion as well as vomiting
biochemical monitoring is needed for these patients
parenteral nutrition
only if other options unsuitable, most hospitals will have team based consultation and collection of baseline measurements before starting
if patients cant absorb food from GI tract may consider, so eg IBD like crohns, or short bowel syndrome due to mesenteric artery infarction
short 1-2 weeks by peripheral veins, or long term via central venous catheter (can stay in for years)
often inpatients but some central catheter peeps admin their own TPN at home, often at night
TPN contains calories, amino acids, vitamins, trace elements; often get a prepackaged TPN mix but sometimes tailored to specific patient
problems: catheter site sepsis so aseptic technique when dealing with the catheter; misplaced catheter can lead to serious problem of extravascular infusion so place under x-ray control; metabolic complications, most common hyperglycaemia esp as background often has inc’d stress level; also hypokalaemia/phosphataemia/magnesaemia, hypercalcaemia/lipidaemia, acid base disorders, re-feeding syndrome, esp be aware for chronic alcoholics starting TPN
metabolic response to injury
ebb phase is short and may correspond to tissue hypoperfusion of shock, with the features an attempt to maintain vascular volume and tissue perfuion, and severity of this phase determines outcome: if severe may develop systemic inflammatory response syndrome which requires intensive life support
in the syndrome get low levels of albumin, zinc, iron, selenium and disordered hormonal regulation eg low T4, TSH, T3
flow phase lasts for days to weeks where metabolism is altered to ensure energy is available for vital tissues at expense of muscle/fat
SIRS criteria: temp >38 or <36, HR >90bpm, resp rate > 20/min, pCO2 < 32mm Hg or on ventilation, WBC count >12,000 or < 4000
biochemical changes in metabolic response to injury: glycogenolysis up so higher circulating glucose, gluconeogenesis up for same reason, lipolysis up to inc level of FFAs and provide glycerol to make glucose, proteolysis up to provide amino acids which can be catabolised for energy or used for wound healing
ebb as energy production decreases, flow as it increases above normal; initially catabolic, then anabolic
acute phase protein response
stimulated by cytokines and raised levels of cortisol + glucagon
C reactive protein and complement increase to combat infection
coagulation factors inc to prevent blood loss
protease inhibitors (alpha antitrypsin, alpha macroglobulin) inc to prevent spread of tissue necrosis when lysosomal enzymes released from damaged cells
misc others with serum amyloid A, haptoglobin, caeruloplasmin up and albumin (redistributed to interstitial fluid), HDL/LDL down
measure serum CRP to monitor inflammatory response on a daily basis as changes rapidly; albumin can also be used
measure erythrocyte sedimentation rate, reflects fibrinogen and Ig conc and changes more slowly so monitor inflam response on weekly timescale
normal serum CRP <3mg/L, if over 100mg/L then person prob has infection which is useful to diagnose bacti infection at early stage in neonates and immunosuppressed patients
GI lab tests
peptic ulcer - serum anti-H pylori Ig titre, urea breath test, biopsy with urease activity testing
pernicious anaemia - serum anti intrinsic factor Ig titre, serum anti parietal cell Ig titre
coeliac disease - IgA antiendomysial Ig titre, anti tissue transglutaminase Ig titre, also screen for IgA deficiency as can give false negatives, duodenal biopsy
small bowel bacterial overgrowth - breath hydrogen test, can also be given alongside lactose or fructose to diagnose malabsorption of these sugars
IBD - faecal calprotectin concentration, serum white cell count and CRP, endoscopy and biopsy to confirm
exocrine pancreatic insufficiency - faecal human pancreas elastase 1 conc, or less reliable chymotrypsin (as degraded in gut)
oftentimes patients with malabsorption recognise this and reduce consumption so will have normal excretion of eg fat bc eat less fatty foods
iron biochemistry
normal adult serum level 10-40micromol/L, but can vary by 50% over 24hrs on circadian rhythm
transferrin saturation usually 30%, if falls to 15% or less then iron deficiency is likely
>30% suggests iron overload
transferrin also reduced during acute phase response and during protein malnutrition
serum ferritin best indicator of total body stores and should be what you use to look for deficiency - conc normally >12microg/L, but increased in acute phase response so marginal deficiency cant be diagnosed off this in such states
zinc protoporphyrin expressed in micromol/ mol haem and usually <60, increases due to iron deficiency or lead exposure
zinc and copper bicohemistry
90% zinc bound to albumin and 10% to a2 macroglobulin
serum zinc conc not useful when CRP >20mg/L as decreases during acute phase response
repeated findings of plasma zinc <5micromol/L suggests impending zinc deficiency
can look for serum copper, though 90% bound to caeruloplasmin, which is greatly increased in the acute phase response
can also look for urinary copper concentration or absorption of Cu65 in patients with low plasma Ca
patients with Wilsons disease will have serum copper <10micromol/L, caeruloplasmin <0.15g/L, urinary copper 5-15micromol/24hrs, liver copper microg/g >250; investigate all young adults with unexplained neurological signs and hepatic disease
prolonged Zn supplementation is common cause of copper deficiency, ask patients with unexplained marrow suppression or neuropathy about dietary supplements
toxicology
often will want serum urea and electrolytes, blood glucose, blood gases, LFTs in every suspected poisoning
plasma conc should be requested for: paracetamol, salicylate, theophylline
if plasma conc rising then absorption still occurring, most likely due to either bolus in gut or correcting hypotension has increased absorption via the portal system
common metal poisonings
lead - inhibits enzymes in haem synthesis pathway so find raised protoporphyrin in RBCs
mercury - organic mercury is highly toxic, can monitor in hair/fingernails for chronic exposure due to eg work; increases in marine life as move up food chain so top predators (tuna, shark) its high and limit intake, esp if pregnant
aluminium - used to treat drinking water so must treat the water used in dialysis as can cross that membrane even though cant cross GIT membrane
arsenic - best indicator is hair analysis
cadmium - typically in industrial workers exposed to its fumes, get nephrotoxicity, bone disease, hepatotoxicity; b2microblobulin in urine can monitor nephron damage; smokers have blood levels 2x non-smokers
cobalt and chromium - concern may get toxicity from metal-on-metal prostheses, so suggested should measure their levels in blood
chronic alcohol abuse
usually from patients history with no highly sensitive and specific markers
elevated gammaGT - in 80% alcohol abusers, but is general sign of liver disease or drugs (phenytoin, pehnobarbital)
hyperuricaemia may occur, also elevated blood triglycerides
>90% patients of chronic alcohol abuse have carb-deficient transferrin
can use these markers to monitor progress as binges will alter their values
myopathy
in all cases of muscle weakness, check serum electrolytes and creatinine kinase as well as full drug history, and exclude history of alcohol abuse
high CK levels in patients who have been convulsing, had a strong electric shock or crush injury
also hyperkalaemia, hyperphosphataemia, and release of fatty acids which may bind Ca giving hypocalcaemia
severe muscle damage may cause acute renal failure by causing shock via sequestering fluid
look for: creatinine kinase, to diagnose; urea and electrolytes to look for renal impairment; alcohol/drugs of abuse, to look for some causes
urine or plasma myoglobin too sensitive, as any minor muscle damage will cause it to be detected
also be aware that serum Mg may be disrupted
miscellaneous reference values
albumin: 35-50g/L
bicarbonate: 22-29mmol/L
chloride: 95-108mmol/L
cholesterol: <5mmol/L
lactate: 0.7-1.8mmol/L
osmolality: 275-295mmol/kg (serum), 50-1400mmol/kg (urine)
paCO2: 4.6-0kPa
pH: 7.35-7.45
paO2: 10.5-13.5
total protein: 60-90g/L
triglyceride: <2.5mmol/L
urea: 2.5-7.8mmol/L
