There are outbreaks of viral, fungal and protozoa diseases among platoons of army soldiers in Indonesia. Soldiers reported sick after 2 weeks of jungle warfare training. It is of concern to the ministry that there are also sporadic reports of avian flu in the nearby villages. In view of these outbreaks, you have been tasked to conduct a pre-mission briefing with blogs and poster to educate future batches of soldiers.
Tuesday, January 22, 2008
MMic dPBL-2: Problem Statement
There are outbreaks of viral, fungal and protozoa diseases among platoons of army soldiers in Indonesia. Soldiers reported sick after 2 weeks of jungle warfare training. It is of concern to the ministry that there are also sporadic reports of avian flu in the nearby villages. In view of these outbreaks, you have been tasked to conduct a pre-mission briefing with blogs and poster to educate future batches of soldiers.
MMic dPBL-2: Possible Viral Infections
References:
- 1. DPDx. (2008). Balantidiasis. Retrieved January 21, 2008, from http://www.dpd.cdc.gov/dpdx/Default.htm >Search: Dengue Fever > Dengue Fever
- 2. DPDx. (2008). Isospora belli. Retrieved January 21, 2008, from http://www.dpd.cdc.gov/dpdx/Default.htm >Search: Japanese Encephalitis> Japanese Encephalitis
- 3. DPDx. (2008). Cryptosporidium. Retrieved January 21, 2008, from http://www.dpd.cdc.gov/dpdx/Default.htm >Search: Tick-borne Encephalitis > Tick-borne Encephalitis
- 4. DPDx. (2008). Trichinellosis. Retrieved January 21, 2008, from http://www.dpd.cdc.gov/dpdx/Default.htm >Search: Hepatitis> Hepatitis
- 5. DPDx. (2008). Ascariasis. Retrieved January 21, 2008, from http://www.dpd.cdc.gov/dpdx/Default.htm >Search: Rabies > Rabies
- 6. DPDx. (2008). Ascariasis. Retrieved January 21, 2008, from http://www.dpd.cdc.gov/dpdx/Default.htm > Search: Avian Flu> Avian Flu
- 7. DPDx. (2008). Ascariasis. Retrieved January 21, 2008, from http://www.dpd.cdc.gov/dpdx/Default.htm > Search: Chikungunya > Chikungunya
Done by: Lizzie, Kent, Gail
MMic dPBL-2: Possible Fungal Infections
(Yeasts)
Subcutaneous Mycoses
Systemic Mycoses
References:
Brooks GF, Carroll KC, Butel JS & Morse SA. 2007. Jawetz, Melnick & Adelberg’s Medical Microbiology, 24th Edition. The McGraw-Hill Companies, Inc.: pages 626-630, 646-648, 650-654.
Gillespie S & Bamford K. 2000. Medical Microbiology and Infection at a Glance. Blackwell Science: pages 70 – 73.
Levinson, W. (2004). Medical Microbiology & Immunology: Examination & Board Review, Eight Edition. The McGraw-Hill Companies, Inc.: pages 331-337.
Done by: Joan Lim & Ye Tun
MMic dPBL-2: Possible Protozoa Infections
References:
1. DPDx. (2008). Balantidiasis. Retrieved
2. DPDx. (2008). Isospora belli. Retrieved
3. DPDx. (2008). Cryptosporidium. Retrieved
4. DPDx. (2008). Trichinellosis. Retrieved
5. DPDx. (2008). Ascariasis. Retrieved
6. DPDx. (2008). Hookworms. Retrieved
7. DPDx. (2008). Trichuriasis. Retrieved
8. DPDx. (2008). Filariasis. Retrieved
9. DPDx. (2008). Hymenolepiasis. Retrieved
10. DPDx. (2008). Schistosomiasis. Retrieved
11. DPDx. (2008). Opisthorchiasis. Retrieved
12. DPDx. (2008). Paragonimiasis. Retrieved
13. GSBS UTMB. (2007). Cestodes. Retrieved
14. GSBS UTMB. (2007). Trematodes. Retrieved
15.
Done by: Adrian Tan & Charmaine Tan
Sunday, December 9, 2007
Group Post: MMIC dPBL-1
Possible Organisms | Biochemical Tests & Results | Culture agars |
Escherichia coli | Gram Stain: - Oxidase: - Lactose: + Lysine: + TSI slant: + Indole: + Methyl red: + VP: - Citrate: - | 5% Sheep Blood Agar Eosin-Methylthionine Blue Agar MacConkey Agar |
Pseudomonas aeruginosa | Gram stain: - Oxidase: + Pyocyanin: + Fluoreseein: + Citrate: + Nitrase: + Lipase: + Ureases: +/- Indole: - TSI: - | Pseudomonas Agar P Pseudomonas Agar F MacConkey agar plate Eosin-Methylthionine Blue agar Cetrimide agar |
Staphylococcus aureus | Gram Stain: + Catalase: + Coagulase: + | 5% Sheep Blood Agar Mannitol Salt Agar |
Staphylococcus saprophyicus | Gram Stain: + Catalase: + Coagulase: - Oxidase: - Phosphatase: - Urease: + Lipase: + 2-hour PYR broth hydrolysis: - D-trehalose; + | 5% Sheep Blood Agar Mueller-Hinton Agar Lawn with Urine |
Enterococcus faecalis | Gram Stain: + Catalase: - | 5% Sheep Blood Agar Chocolate Agar 6.5% NaCl Concentration Agar (halotolerant) Bile Esculin Agar (BEA) Slants |
Proteus-Providencia-Morganella e.g. Proteus mirabilis | Gram Stain: - Lactose: - Oxidase: - Urease: + Indole: - Nitrogen Reductase: + Methyl Red: + Vogues-Proskauer: - Catalase: + Phenylalanine Deaminase: + | MacConkey Agar CLED |
Klebsiella-Enterobacter-Serratia e.g. Klebsiella oxytoca | Gram Stain: - Oxidase: - Lactose: - Urease: - Nitrate: + Indole: + Sucrose: + Dextrose: - Methyl Red: - Vogues-Proskauer: + Gelatin: - Amylase: + Lipase: - Catalase: + Citrate: + H2S: - | 5% Sheep Blood Agar MacConkey Agar Trypticase Soy Agar |
Klebsiella-Enterobacter-Serratia e.g. Klesiella pneumoniae | Indole: - Sucrose: + Dextrose: - Methyl Red: - Vogues-Proskauer: + H2S: - Lysine Decarboxylase: + | 5% Sheep Blood Agar MacConkey Agar |
Done by Lizzie
Campylobacter jejuni | Oxidase (+) Sensitivity to nalidixic acid | Blood agar with antibiotics to other fecal microbes Fail to grow at 25C, must grow at 42C Microaerophilic (grow at 5% oxygen + 10% carbon dioxide) |
Shigella species | TSI (alkaline slant and acidic butt, with no gas and no H2S) | MacConkey's or EMB agar (colourless colonies) Anaerobe |
Escherichia coli | TSI (acidic slant and acidic butt, with gas ,but no H2S) Indole test (+) | MacConkey's (pink colonies) or EMB agar (green sheen colonies) Anaerobe |
Clostridium perfringens | ELISA to detect Clostridium perfringens Enterotoxin | TSC agar containing egg yolk Neomycin blood agar Strict anaerobe |
Clostridium difficile | Cytotoxicity test on human cell-line ELISA to detect Clostridium perfringens exotoxin | Cycloserine cefoxitin fructose egg yolk agar (CCFA) |
Vibrio Cholerae | Oxidase (+) TSI (acidic slant acidic deep; no gas) Agglutination of bacteria by polyvalent O1 antibody | TCBS (yellow colonies), Blood agar |
Vibro parahemolyticus | Oxidase-positive Grow in 10% NaCl | TCBS (green colonies) |
Bacillus cereus | Lecithinase (+) Motile (swarming on agar) Catalase (+) Indole (-) Nitrate (+) Urease (-) Citrate (+) MR (-) VP(+) TSI (Alkaline slant, acidic deep) Oxidase (-) Lactose fermenting; sucrose non-fermenting | Sheep’s blood Agar (beta hemolytic), large greenish colonies), Mannitol (grey colonies) |
Yersinia enterocolitica | Motile at 25 deg MR (+) VP (-) Urease (-) Indole (-) Citrate (-) Catalase (+) TSI (acidic slant acidic deep; no gas) | MacConkey Agar (small lactose negative colonies) EMB |
Staphylococci aureus | Catalase (+) Citrate (-) Coagulase (+) Indole (-)ME (+) Oxidase (-) Urease (-) VP (-) TSI (acidic slant acidic deep) | Sheep blood agar (beta hemolytic, yellow colonies) Mannitol salt agar (yellow colonies) 7.5% NaCl media |
Done by Kent and Ye Tun
E. coli | Oxidase - Catalase + Indole + Methyl red + Voges-Prokauer - Citrate Utilization - Urease - TSI; H2S production Acid slant/ acid butt & gas produced; + | MacConkey agar Lactose fermenters; (red colonies) Blood agar β-hemolytic |
Klebsiella sp (Klebsiella pneumuniae, Klebsiella oxytoca) | Oxidase - Catalase + Indole + Methyl red - Voges-Prokauer + Citrate Utilization + Urease - TSI; H2S production Alkaline slant/ acid butt; - | MacConkey agar Lactose fermenters; (pink colonies) Blood agar Mucoid colonies formed |
Enterobacter (Enterobacter aerogenes) | Oxidase - Catalase + Indole - Methyl red - Voges-Prokauer + Citrate Utilization + Urease TSI; H2S production | MacConkey agar Blood agar |
Serratia | Oxidase - Catalase + Indole Methyl red - Voges-Prokauer + Citrate Utilization + Urease TSI; H2S production | MacConkey agar Blood agar |
Proteus mirabilis | Oxidase - Catalase + Indole + Methyl red + Voges-Prokauer - Citrate Utilization Urease + TSI; H2S production Alkaline slant/ acid butt; + | MacConkey agar Non-lactose fermenters; (white colonies) Blood agar Swarming observed |
Morganella morganii | Oxidase - Catalase + Indole Methyl red Voges-Prokauer Citrate Utilization Urease + TSI; H2S production | MacConkey agar Non-lactose fermenters; (white colonies) Blood agar |
Pseudomona aeruginosa | Oxidase + Catalase + Indole - Methyl red Voges-Prokauer Citrate Utilization + Urease Coagulase TSI; H2S production | MacConkey agar Non-lactose fermenters; (white colonies) Blood agar |
Staphylococcus aureus | Catalase + Methyl red Voges-Prokauer Citrate Utilization Coagulase + | MacConkey agar - Blood agar β-hemolytic |
Staphylococcus saprophyticus | Catalase + Methyl red Voges-Prokauer Citrate Utilization Urease + Coagulase - | MacConkey agar Non-lactose fermenters; (white colonies) Blood agar - |
Enterococcus sp | Catalase Indole Methyl red Voges-Prokauer Citrate Utilization Urease Coagulase TSI; H2S production | MacConkey agar Blood agar |
Done by Joan
Possible Organisms | Biochemical Tests & Results | Culture agars |
Streptococcus pneumoniae | Gram Staining +ve Catalase -ve | 2. 5% Sheep Blood agar with Optocin sensitivity Alpha-hemolysis with sensitivity to optocin |
Klebsiella-Enterobacter-Serratia (Klebsiella sp.) | Gram Staining -ve Oxidase -ve Indole +ve/-ve MR-VP +/- or -/+ Citrate +ve H2S Production -ve Lysine Decarbocylase +ve | Lactose Fermentation (MacConkey agar) +ve |
Bordetella pertussis | Gram Staining -ve Oxidase +ve Urease -ve Nitrase -ve Citrate -ve | Borde+ - Gengou agar BCYE agar |
Done by Charmaine
Staphylococcus aureus | Gram Staining + Catalase + Coagulase + | Mannitol Salt Agar with 7-9% NaCl Mannitol fermentation Sheep Blood Agar Hemolysis of blood agar |
Streptococcus pyogenes | Gram Staining + Catalase - PYR (Pyrrolidonearylamidase) Test + | Sheep Blood Agar Beta Hemolysis |
Enterococci | Gram Staining + Bile- esculin test - | Sheep Blood Agar Gamma Hemolysis 6.5% NaCl Agar Can grow with 6.5% Nacl |
Escherichia coli | Gram Staining - Indole + Methyl Red + Voges Prokauer - Citrate - TSI Acid slant and butt with gas production but no H2S in TSI | MacConkey Lactose fermentative (deep red colony) Eosin Methylene Blue( EMB) Green metallic sheen on EMB |
Pseudomonas aeruginosa | Gram Staining - Oxidase + Catalase + Indole + Citrate + Urease - TSI - Pyocyanin - Fluroscein - | MAC Non- lactose fermentative (colourless colony on MAC) Pseudo F agar PositiveFluorescein pigment in Pseudo F agar |
Done by Ai Tee
Gardnerella vaginalis | Catalase - Oxidase - Vaginal pH test pH of vaginal fluid >5.0 Whiff test Release of bad-smelling odor when discharge is mixed with KOH DNA Probe (Affirm™ VP III) High concentrations of G. vaginalis in VP III test | Chocolate agar Small, circular, convex, gray colonies HBT agar Colistin-oxolinic acid blood agar Beta-hemolysis |
E. coli | Triple Sugar Iron (TSI) test TSI test show acid slant, acid butt with gas. Indole test + Methyl red test + Voges-Proskauer (VP) test - Citrate test - | EMB agar Black colonies with greenish-black metallic sheen on EMB agar MacConkey agar Deep red colonies on MacConkey agar (lactose fermentor) TSI agar |
Pseudomonas aeruginosa | Oxidase test + Catalase test + Nitrase tese + Lipase test + TSI test Metallic sheen growth on the surface of TSI agar | EMB agar Mac Conkey’s agar (sterile specimen) Cetrimide agar (non-sterile specimen) Colonies appear flat, large, and oval. Secretes blue-green pigment (pyocyanin) on Cetrimide agar TSI agar No colour change on TSI medium (K/K/g-/H2S- profile) |
Neisseria gonorrhoeae | Oxidase test + Direct immunofluorescence | Thayer-Martin chocolate agar Growth of spherical shape colonies on TM chocolate agar |
Chlamydia Trachomatis | Direct immunofluorescence Presence of monoclonal antibodies (mAbs) to epitopes in the VS4 region of MOMP ELISA Presence of Chlamydia antigens in ELISA test PCR DNA-based tests. | Blood agar Mac Conkey’s agar |
Trichomonas vaginalis | Vaginal pH test pH of vaginal fluid >4.5 PCR Enzyme immunoassay Presence of T. vaginalis antigen in enzyme immunoassay Direct Immunofluorescence. | Blood agar Mac Conkey’s agar |
Mobiluncus | Catalase test - Indole test - Oxidase test - | Blood agar Clear, colourless colonies, around 2mm. Mac Conkey’s agar |
Enterobacter-Klebsiella-Serrtia | - | Blood agar Mac Conkey’s agar Deep red colonies on MacConkey agar (lactose fermentor) |
Proteus-Providencia-Morganella | Urease test + Indole test + | Blood agar Appear as swarming on blood agar. Mac Conkey agar Colourless colonies on MacConkey agar (Non-lactose fermentor) |
Enterococcus faecalis | Catalase test - | Blood agar Alpha, beta, or non-hemolytic on blood agar. Mac Conkey agar |
Candida albicans | - | Blood agar Sabouraud’s agar Presence of white, butyrous colonies. |
Staphylococcus saprophyticus | Catalase test + Coagulase test - Urease test + Lipase test + | Blood agar Mac Conkey’s agar Spherical, irregular grape-like cluster in culture. |
Done by Adrian
Sunday, December 2, 2007
Group Post: MMIC dPBL 1
Diagnosis | Possible bacteria species | Morphology & Microscopy | |
Case 1 | Urinary Tract Infection (UTI) | Staphylococcus aureus Staphylococcus saprophyticus Enterococcus faecalis | Gram Positive, Spherical-shaped, immobile and form grape-like clusters Gram Positive, Cocci-shaped occur singly and in pairs, short chains, and grape-like clusters Gram Positive, Cocci-shaped in pairs and chains |
Case 2 | Enterocolitis | Campylobacter jejuni Shigella species Salmonella species Escherichia coli (entero-pathogenic; EPEC) Clostridium perfringens Clostridium difficile Vibrio Cholerae Vibro parahemolyticus Bacillus cereus Yersinia enterocolitica Staphylococci aureus | Gram-negative rods Gram-negative rods Gram-negative rods Gram-negative rods Gram-positive rods Gram-positive rods Gram-negative rods Gram-negative rods Gram-positive rods Gram-negative rods Gram-positive cocci |
Case 3 | Urinary Tract Infection (UTI) | E. coli Klebsiella sp (Klebsiella pneumuniae, Klebsiella oxytoca) Enterobacter (Enterobacter aerogenes) Serratia Proteus mirabilis Morganella morganii Pseudomona aeruginosa Staphylococcus aureus Staphylococcus saprophyticus Enterococcus sp. | Gram negative bacilli Gram negative bacilli Gram negative bacilli Gram negative bacilli Gram negative bacilli Gram negative bacilli Gram negative bacilli Gram positive cocci Gram positive cocci Gram positive cocci |
Case 4 | Bronchitis | Strep. Pneumoniae Klebsiella-Enterobacter-Serratia (Klebsiella sp.) Bordetella pertussis | Gram positive, diplococci, lancet-shaped Gram negative rods Gram negative, coccibacilli |
Case 5 | Wound Infection | Beta Haemolytic Streptococci (Streptococcus pyogenes) Enterococci (Enterococcus faecalis) Staphylococci (Staphylococcus aureus/MRSA) Clostridium Pseudomonas aeruginosa Enterobacter species Escherichia coli Klebsiella species Proteus species Bacteroides | Gram-positive facultative anaerobes cocci Gram-positive facultative anaerobes cocci Gram-positive facultative anaerobes cocci Gram-positive anaerobe rod Gram-negative aerobic rods Gram-negative facultative rods Gram-negative facultative rods Gram-negative facultative rods Gram-negative facultative rods Gram negative Anaerobes rods |
Case 6 | Urinary Tract Infection (UTI) | Gardnerella vaginalis Chlamydia trachomatis Neisseria gonorrhoeae Ureaplasma urealyticum Staphylococcus saprophyticus Enterobacteriaceae (E. coli, serratia, klebsiella, enterobacter, citrobacter) Enterococci | Gram-negative rod with a gram-positive cell wall Gram-negative bacteria, coccoid or rod-shaped Gram negative cocci Gram-negative bacteria Gram positive, globular shaped, colonies resemble grape-like clusters Gram-negative, rod-shaped Gram-positive cocci which often occur in pairs (diplococci) |
Brief description of diagnosis:
1. Urinary Tract Infection (UTI)
A urinary tract infection (UTI) is a bacterial infection that affects any part of the urinary tract. The urinary tract is made up of the kidneys, ureters, bladder, and urethra, and each plays a role in removing liquid waste from the body. Although urine contains a variety of fluids, salts, and waste products, it usually does not have bacteria in it. When bacteria get into the bladder or kidney and multiply in the urine, they cause a UTI.
Cystitis: bladder infection.
Pyelonephritis: kidney infection is much more serious.
Urethritis: urethra infection.
Each type of UTI may result in more specific signs and symptoms, depending on which part of your urinary tract is infected. The symptoms shown in acute pylonephritis are closest to what is given in the case study, whereby it is an infection of your kidneys may occur after spreading from an infection in your bladder. Kidney infection can cause upper back and flank pain, high fever, shaking chills, and nausea or vomiting.
Pyelonephritis is an inflammation of the kidney and upper urinary tract that usually results from noncontagious bacterial infection of the bladder (cystitis). In acute pylonephritis, bacteria begin colonising the tubules and connective tissue of the kidney itself. Small abscesses and streaks of pus begin to appear in the renal cortex and medulla respectively. Pyelonephritis most often occurs as a result of urinary tract infection, particularly in the presence of occasional or persistent backflow of urine from the bladder into the ureters or kidney pelvis (vesicoureteric reflux).
Urinary catheterization is the insertion of a catheter into a patient's bladder through the urethra, to drain urine from the bladder into an attached bag or container. Indwelling catheters should be restricted to patients whose incontinence is caused by urinary tract obstruction that can not be treated, and for which alternative therapy is not feasible. However, the catheter may introduce bacteria into the urethra and bladder, resulting in urinary tract
infection. The risk for developing a UTI increases when long-term catheterization is required.
Bacteria causing UTI may be isolated from the vaginal discharge. UTI can also result from sexual intercourse where bacteria from the vaginal area is transferred to the urethra and bladder.
Authors: Lizzie, Joan and Adrian
2. Enterocolitis
Enterocollitis is the inflammation of the small intestine and colon. Disease-causing bacteria usually invade the small intestines and colon and cause inflammation (blood or pus in the stool, fever) and abdominal pain and diarrhea. Campylobacter jejuni is the most common bacterium that causes acute enterocolitis in the U.S. Other bacteria that cause enterocolitis include Shigella, Salmonella and EPEC (E. Coli Enteropathogenic). These bacteria usually are acquired by drinking contaminated water or eating contaminated foods such as vegetables, poultry, and dairy products.
Enterocolitis caused by the bacterium Clostridium difficile is unusual because it often is caused by antibiotic treatment. It is also the most common nosocomial infection (infection acquired while in the hospital) to cause diarrhea.
E. coli O157:H7 produces a toxin that causes hemorrhagic enterocolitis (enterocolitis with bleeding). There was a famous outbreak of hemorrhagic enterocolitis in the U. S. traced to contaminated ground beef in hamburgers (hence it is also called hamburger colitis).
Bacterial overgrowth of the small intestine.
Because of small intestinal problems, normal colonic bacteria may spread from the colon and into the small intestine. When they do, they are in a position to digest food that the small intestine has not had time to digest and absorb. The mechanism that leads to the development of diarrhea in bacterial overgrowth is not known.
Authors: Kent and Ye Tun
3. Bronchitis
Bronchitis is an inflammation of the bronchi of the lungs. There are two main types of bronchitis: acute and chronic bronchitis.
Acute bronchitis can be caused by viruses such as influenza A and B, parainfluenza virus, respiratory syncytical virus, coronavirus, adenovirus, and rhinovirus. Bacterias are also able to cause acute bronchitis. Air pollutants such as cigarette smoke, dusts, or fumes of chemicals are also able to cause acute bronchitis.
The symptoms of acute bronchitis include:
* Sore throat
* Chest congestion
* Sinus fullness
* Breathlessness
* Wheezing
* Slight fever and chills
* Overall malaise
* Cough
Chronic bronchitis is where by the signs and symptoms become prolonged, and is defined clinically as a
persistent cough that produces sputum (phlegm) and mucus, for at least three months in two consecutive years. Chronic bronchitis is part of chronic obstructive pulmonary disease (COPD).
Author: Charmaine
4. Surgical Site Infection (SSI)
There are different levels of SSI:
• Superficial incisional, affecting the skin and subcutaneous tissue.
• Deep incisional, affecting the facial and muscle layers.
• Organ or space infection affecting any part of the anatomy opened or manipulated during the operation.
Symptoms: Pain, fever, inflammation, swelling and pus formation
Causes of infection:
-Microbes contamination (most commonly by S. aureus, S. pyogenes or P. aeruginosa
-Migration of patients’ own bacterial normal flora
-The materials or equipment used in the operational procedures (for e.g.: poor surgical techniques)
Author: Ai Tee
Wednesday, November 7, 2007
19th Week of SIP - Medical Microbiology
I am sharing about a very scary virus infection. It causes hepatitis and has mortality rate of 30 percent. Quite scary huh!
We all heard about hepatitis B. But the motality rate of hepatitis B is not that high (0.5 - 1%). However this particular virus is somehow related to hepatitis B virus and it depends on the hepatitis B virus to infect the host.
Hepatitis D Virus
This interesting virus is the Hepatitis D Virus (HDV). HDV is a single-stranded RNA virus and it has HDAg as a enveloped protein (enclosing the RNA) and HBsAg (derived from hepatitis B virus) as its surface antigens. Without the HBsAg coating, HDV cannot infect, replicate or express on its own. The famility of HDV has not been identified.
Route of transmission and symptoms are similar to those of HBV. But it is more severe. It was reported that 70-80% of chronic HBV carriers with HDV superinfection develope chronic liver disease wuth cirrhosis.
As I mentioned HDV need HBV, thus the disease occur when HDV either superinfects the chronic HBV carriers or coinfect the person together with HBV. Symptoms are more severe in superinfection cases.
There is no vaccine for HDV and the best way to prevent it is to avoid risk behaviors. Immunization to HBV can avoid coinfection.
Okay...so much about the background and now I am sharing the laboratory diagnosis of HDV.
HDV is diagnosed serologically by detecting total antibodies to HDAg (delta antigen) by Competitive ELISA. The principle of the essay is as below. Enjoy....
Anti-HD present in the sample and labelled anti-HD antibodies compete for a fixed number of HDAg on the surface (sold-phase). The enzyme-labelled anti-HD gives colour after adding substrate, and the concentration of enzyme-labelled anti-HD can be worked out in relation to the O.D. reading. The amount of labelled anti-HD measured is inversly propotional to the concentration of anti-HD present in the samples.